[Backend] redo the work for "Areas of Focus"

[Pomax]

redpen: https://redpen.io/p/bn82314af4898269e1

The areas of focus backend needs to be modified a little bit to match a design change: each area of focus should have an optional link to an internal page, which means we need the following backend changes:

• [x] un-mixin the area of focus because it no longer needs UniversalLink functionality
• [x] add a page foreignkey to Wagtail.Page with a PageChooser, with null=True/blank=True

And probably:

• [x] rename the AreaOfFocus model to HomepageFocusAreas
• [x] audit the code to see if we should delete the UniversalLink code again. If nothing else uses it, it might not make sense to keep it around
• [x] add factory code for the homepage that builds the three areas of focus and links them:

Empower Action

image: source/images/homepage/area-of-focus/01_empower-action-icon.png description: Issues like privacy, trustworthy AI, and digital rights impact all of us. Mozilla helps empower meaningful

Connect Leaders

image: source/images/homepage/area-of-focus/02_connect-leaders-icon.png description: We support activists and thought leaders shaping the future of our online lives.

Investigate & Research

image: source/images/homepage/area-of-focus/03_investigate-research-icon.png description: We publish open source research and host global convenings to make ideas like trustworthy AI mainstream.

[Pomax]

Frontend work for this has mostly been done already in https://github.com/mozilla/foundation.mozilla.org/pull/4990, and should work with the changes effected by this issue's work.

[mmmavis]

~Can this request be addressed as part of this ticket as well? 🥺~

[added to list]

[KalobTaulien]

We need a new reviewer since Pomax and I have both been working on this PR. @mmmavis any chance you have time to review this?

Note: I need to rebase this branch again.

[Pomax]

https://github.com/mozilla/foundation.mozilla.org/pull/4990 got merged in.