global: check for extension initialization before registering breadcrumb

There are scenarios where some flask extension might want to provide `Flask-Breadcrumbs` capabilities for its blueprints, but the user of the extension is not interested in this (e.g. if the application doesn't have any UI views, but only provides a REST API).

It should be possible to conditionally register the breadcrumb, by checking if `current_breadcrumbs` exists before performing the registration:

```python
# inside "register_breadcrumb"
...
def breadcrumb_decorator(func):
"""Applie standard menu decorator and assign breadcrumb."""
if current_breadcrumbs:
func.__breadcrumb__ = func_path
return menu_decorator(func)
...
```

(this change depends on https://github.com/inveniosoftware/flask-menu/issues/65)

Start by locating register_breadcrumb and review the linked flask-menu issue 65, since this change depends on it. Confirm how current_breadcrumbs is exposed before deciding where the conditional check belongs; done means breadcrumb registration can be skipped when it is unavailable while existing registration still works.

Written by the indexing model from the issue text.