Add basic OIDC integration with locally setup Authentik

[kmoy1]

NOTE: Since this requires local manual setup of an OIDC IDP, this should not be merged into master but into its own branch (e.g. 'OIDC').

Addresses #1797

Proposed Changes

• Added a very basic form of OIDC Authentication support, by adding a "Login with Authentik" button to the Login page. It will authenticate with another (locally) spun-up Authentik IDP server, instead of requiring username/password combo. It will use the email of the user instead.

Please check that the PR fulfills these requirements

• [] Tests for the changes have been added (for bug fixes / features) - not sure how to test this, because this code review requires manual setup of an OAuth2 provider.
• Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR
  (e.g. database migration)?

Yes. Users need to spin up an Authentik application of their own and then set these environment variables in the Django app (or hardcode them in settings_global.py: not recommended), values of which are provided by Authentik.

OIDC_RP_CLIENT_ID
OIDC_RP_CLIENT_SECRET
OIDC_OP_AUTHORIZATION_ENDPOINT
OIDC_OP_TOKEN_ENDPOINT
OIDC_OP_USER_ENDPOINT
OIDC_OP_JWKS_ENDPOINT

They also need to set
OIDC_RP_SIGN_ALGO=RS256

[Keridos]

This looks like a draft for me. Under the assumption that the implementation is working:
The OIDC login should be only activated when the OIDC Variables are set.
Additional Choice: When it is activated you may disable regular account login with username and password.
Additionally it would be very nice to make the name of the OIDC Provicer (on the button) also configurable via env variable.

Sadly I lack the skills for web development and never worked with Django so far or I'd implement the changes myself.