Anonym Part 4: Navigation

[dchukhin]

If this changeset needs to go into the FXC codebase, please add the WMO and FXC label.

This pull request depends on #16962

One-line summary

Add an editable navigation bar to anonym pages.

Significant changes and points to review

Similar to advertising pages, the AnonymIndexPage now has a navigation field, which allows users to set what will appear in the navbar. Because the logic is so similar, I factored it out into a SubNavigationMixin, which both the AdvertisingIndexPage and the AnonymIndexPage now use.

Issue / Bugzilla link

1. log in to Wagtail
2. find or create an Anonym Index Page
3. go to the 'Settings' tab to edit the navigation to match the figma designs from the JIRA ticket. Note: this pull request does NOT include the styles needed to match the designs

[codecov]

Codecov Report

❌ Patch coverage is 89.06250% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.72%. Comparing base (2cb9951) to head (e6a380c).

Files with missing lines	Patch %	Lines
bedrock/mozorg/models.py	84.09%	7 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           anonym-index-content-subpage   #16964   +/-   ##
=============================================================
  Coverage                         80.72%   80.72%           
=============================================================
  Files                               164      164           
  Lines                              9290     9328   +38     
=============================================================
+ Hits                               7499     7530   +31     
- Misses                             1791     1798    +7     

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