Bug 2040735 - Show FxA email in signed in banner

[fterdal]

We can get the user's email from the FxA social account (under "extra_data"). The email should always be available, but we can still show their FxA id if not.

[fterdal]

For QA, I used the local fake FxA OAuth provider. Here are the steps:

1. Run the app locally, and go to http://localhost:8000/accounts/dummy/login/
2. Fill in a user id that already exists in the database (use make seed to create some if you want).
3. Fill in the email and submit.
4. You'll be taken to the interstitial /auth-complete page with a loading skeleton. Manually navigate to http://localhost:8000/dev-login/
5. You should see the banner that says "Successfully signed in as [email]"

This banner comes from Django's all-auth package. In general, users should always have an email associated with their account, but if they don't for some reason, we default to their id.

[nathan-barrett]

r+ but check in with Dasha if this works for what she was thinking

[nathan-barrett]

re-approving to get that fxshaving-dev approval