MPP-3698: Enable mypy --check-untyped-def --no-implicit-reexport across code

[jwhitlock]

For MPP-3698, I updated the code so both --check-untyped-def and --no-implicit-reexport is used across our code. This means that code without type annotations is checked by mypy rather than skipped, and finds errors like not considering None or other alternate return types.

This is a big PR, but the changes themselves are small.

Bugs Fixed

• api/authentication.py
  • introspect_token - Error handler referred to fxa_resp, was not initialized (Sentry RELAY-J0, RELAY-J2)
• api/permissions.py
  • HasPremium called a bool (unused in code)
• emails/management/commands/process_emails_from_sqs.py
  • Command.handle - Use different name for exception inside of an exception handler

Code changes

• Moved app config helper functions to app file. For example emails_config() is now in emails.app.py.
• Lots of new annotations
• Add assert to ensure a value is not None when mypy complained
• Add isinstance to ensure request.user is not an anonymous user in API code
• Add hasattr checks as needed for dynamically added fields
• Comments and long code lines wrapped to 88 chars (in changed files). Long function names have a # noqa: E501 comment.
• Used walrus operator (:=) to assign and assert (usually not None) in one line
• Changes Optional[X] to Python 3.9 format X | None
• More use of from __future__ import annotations for PEP-563, Postponed Evaluation of Annotations, to allow more forward annotations without quotes.
• api/tests/authentication_tests.py
  • Used instance of FxaTokenAuthentication instead of class in FxaTokenAuthenticationTest
  • Used Django 4.2 header-setting style in FxaTokenAuthenticationTest
• privaterelay/settings.py
  • RELAY_CHANNEL is now restricted to "local", "dev", "stage", or "prod"
  • Added many annotations to privaterelay/settings.py for "circular import" issues.
  • Removed cast=str when it is the default.
  • Set AUTO_RELOAD_BUNDLES = False to avoid django-ftl warning

[jwhitlock]

PR #4321 is merged, this is ready for review.

[groovecoder]

One blocking question about returning 401 status code that's very important to get right because Firefox clients change their behavior based on that status code.

[jwhitlock]

Thanks for the quick review @groovecoder! I've answered some questions, and made two suggested changes:

• Removed Optional from privaterelay/settings.py
• Added create_free_phones_flag_for_user helper

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ UnicodeEncodeError: 'utf-8' codec can't encode characters in position 404-406: surrogates not allowed emails.utils in ses_send_raw_email View Issue

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