Get feature flags from django-waffle in Accounts

[davinotdavid]

What changed?

• Added a GET /auth/waffle-flags endpoint that makes a request to Accounts to retrieve the feature flags.
• In the frontend, during the post login process (PostLoginView component), we are making the call to the waffle flags and storing just the flags in the user store.

Why?

Now that we are exposing the django-waffle feature flags from Accounts, we should be able to make them readily available to be used.

How to test

Prerequisite:

• For this, you need to run thunderbird-accounts using this PR and appointment together so that the flags set in Accounts can be shown here.
• VueJS DevTools installed

[thunderbird-accounts]

• Login as admin@example.org / admin and navigate to http://localhost:8087/admin/waffle/flag/ and add an arbitrary flag there.

[appointment]

• Open your VueJS DevTools
• Login as admin@example.org / admin
• No need to go through FTUE, just check the GET /auth/waffle-flags call in the Network tab and check the Pinia store in the VueJS DevTools in the user store to check that the featureFlags key is populated correctly.

Limitations and Notes

When we are using non-authenticated requests, we are not able to feature flag per user (of course). This is a problem for the Google Calendar Invites (currently GOOGLE_INVITE_ENABLED env var) as this feature flag is required in the unauthenticated booking endpoints, unfortunately.

Applicable Issues

Closes #1737
Related with thunderbird/thunderbird-accounts#1072

Screenshots