Install django-waffle for per-user and per-group feature flags.

[rebeccacremona]

See ENG-390.

I experimented with this locally: it works just like it says on the label:

• I created a flag in the Django admin, and then experimented with checking it from a view, from a template, and within a page's JS, and each time, it worked on my first try.
• I created a group, added myself to that group, and then checked all the same things.
• I used the admin to "override" the user-based settings, and it worked too.

I think this will work great for our needs. There is even an easy way to expose the results as JSON, if that proves more convenient for tomorrow's JS.

The only question will be, what is our workflow for creating the flags in the DB. I'm thinking probably, any PR that starts using a new flag should include a data migration for creating that flag.

Note: I don't think this comes with a built in solution for, "Turn this flag on for all org users associated with Registrar X", but if that sort of situation comes up, we can write utilities for it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4ff85e6) 70.99% compared to head (5cb14c6) 70.98%. Report is 6 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3455 +/- ## =========================================== - Coverage 70.99% 70.98% -0.01% =========================================== Files 48 48 Lines 6465 6463 -2 =========================================== - Hits 4590 4588 -2 Misses 1875 1875 ```

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