Make the Custom Navbar/Footer in Pro Template work in Multi-Site Installs

#### Is your feature request related to a problem? Please describe.

We receently implemented WagtailCRX, and noticed that wagtail's Navbar and Footer models in the basic template work in multi-site installations, but the custom Navbar and Footer models in the pro template do not (though you could fall back to wagtail's default models with `CRX_DISABLE_NAVBAR = False` and `CRX_DISABLE_FOOTER = False`).

#### Describe the solution you'd like

We implemented the following in our site, which allows us to use custom Navbar(s) and Footer(s) in our multi-site install. Because nulls are allowed, it won't cause errors on existing installs. In existing installs, though, the template tags as implemented below would cause the components to not be rendered (existing model instances would not associated with any site initially). To resolve this, we could:

- Modify the template tags to check for a setting before implementing their site checks so it won't disappear the navbars and footers in existing projects (falling back to current functionality if `CRX_USE_SITE_FIELD=False` for example)
- Or, just set the template tags to fall back to showing all Navbar(s) and Footer(s) if there are no site-specific results

Would a PR to update the pro template (and add relevant tests & updates to docs) be of interest? Any additional thoughts/recommendations?

This proposal would also resolve #673

---

models.py

```python
"""
Create or customize your page models here.
"""

from coderedcms.blocks import (
HTML_STREAMBLOCKS,
LAYOUT_STREAMBLOCKS,
BaseBlock,
BaseLinkBlock,
LinkStructValue,
)
from coderedcms.forms import CoderedFormField
from coderedcms.models import (
CoderedArticleIndexPage,
CoderedArticlePage,
CoderedEmail,
CoderedEventIndexPage,
CoderedEventOccurrence,
CoderedEventPage,
CoderedFormPage,
CoderedLocationIndexPage,
CoderedLocationPage,
CoderedWebPage,
)
from django.db import models
from modelcluster.fields import ParentalKey
from wagtail import blocks
from wagtail.admin.panels import FieldPanel
from wagtail.fields import StreamField
from wagtail.snippets.models import register_snippet

# Other models...

@register_snippet
class Navbar(models.Model):
"""Custom navigation bar / menu."""

class Meta:
"""Meta class for Navbar."""
verbose_name = "Navigation Bar"

name = models.CharField(
max_length=255,
)
content = StreamField(
[
("link", NavbarLinkBlock()),
("dropdown", NavbarDropdownBlock()),
],
use_json_field=True,
)

site = models.ForeignKey( # <-- New Field
"wagtailcore.Site",
on_delete=models.CASCADE,
related_name="navbars",
null=True,
blank=True,
)

panels = [
FieldPanel("name"),
FieldPanel("site"), # <-- New panel item
FieldPanel("content"),
]

def __str__(self) -> str:
return self.name

@register_snippet
class Footer(models.Model):
"""Custom footer for bottom of pages on the site."""

class Meta:
"""Meta class for Footer."""
verbose_name = "Footer"

name = models.CharField(
max_length=255,
)
content = StreamField(
LAYOUT_STREAMBLOCKS,
verbose_name="Content",
blank=True,
use_json_field=True,
)

site = models.ForeignKey( # <-- New Field
"wagtailcore.Site",
on_delete=models.CASCADE,
related_name="footers",
null=True,
blank=True,
)

panels = [
FieldPanel("name"),
FieldPanel("site"), # <-- New panel item
FieldPanel("content"),
]

def __str__(self) -> str:
return self.name
```

Updated template tags render the navbar(s) and footer(s) for the current site:

website_tags.py

```python
"""Custom template tags for the website app."""
from django import template
from wagtail.models import Site
from website.models import Footer, Navbar

register = template.Library()

@register.simple_tag(takes_context=True)
def get_website_navbars(context):
"""Get the navbars for the current site.

Args:
context: The template context which contains the current request

Returns:
QuerySet: Navbar queryset filtered by the current site
"""
try:
# Get the current request from context
request = context['request']
# Get the current site from the request
current_site = Site.find_for_request(request)
# Return navbars associated with the current site
return Navbar.objects.filter(site=current_site)
except (KeyError, AttributeError):
# Fallback to returning all navbars if we can't determine the current site
return Navbar.objects.all()

@register.simple_tag(takes_context=True)
def get_website_footers(context):
"""Get the footers for the current site.

Args:
context: The template context which contains the current request

Returns:
QuerySet: Footer queryset filtered by the current site
"""
try:
# Get the current request from context
request = context['request']
# Get the current site from the request
current_site = Site.find_for_request(request)
# Return footers associated with the current site
return Footer.objects.filter(site=current_site)
except (KeyError, AttributeError):
# Fallback to returning all footers if we can't determine the current site
return Footer.objects.all()
```

Comienza revisando las definiciones personalizadas de Navbar y Footer en models.py y las funciones de renderizado en website_tags.py; después, inspecciona las pruebas existentes de plantillas pro y la documentación. Define cómo deben comportarse los resultados específicos del sitio y las instancias existentes sin asignar, añade la migración y la cobertura requeridas, y actualiza la documentación cuando los componentes multi-sitio se rendericen correctamente sin romper las instalaciones existentes.

Escrito por el modelo de indexación a partir del texto del issue.