Content manageable footer

### As a Joplin admin, I want the ability to manage pages on the site's homepage and footer, so that residents can access information that's global to the whole site.

**Acceptance criteria**
Designs for:
- [ ] Admin can manage footer links via Joplin
- [ ] Admin can decide which services to highlight on the homepage

ideas from @briaguya:
- New snippet for footer in Joplin (or maybe a "Home" page?, we have some options)
- Create a new topic/topic collection for pages we want in the footer
- Create pages as normal and use those topics

**Is your feature request related to a problem? Please describe.**
We need to publish pages to the footer of the site. Currently, we have the [Privacy Policy](https://alpha.austin.gov/es/preview/information/UGFnZVJldmlzaW9uTm9kZToxMzcx), and soon we'll have "Legal notice" and "Council message board"; I'm sure more will come in the future.

Currently, "Privacy policy" doesn't publish out because it isn't associated to a topic or a department. Per #2862, we would expect the URL to be alpha.austin.gov/privacy-policy, and it wouldn't need to be associated to a topic or a department, because it is global to the entire site.

**Describe the solution you'd like**
- Somewhere in Joplin where an admin can select pages for the footer
- Footer pages should publish out even if they aren't associated with
Not sure what the implications are for our IA; need to work this out with dev.

**Describe alternatives you've considered**

**Additional context**