Allow setting SSO config fully in env vars and support multiple public OIDC providers

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a description of your work suitable for publishing on our forum
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Adds front-end support for multiple OIDC SSO providers.

Notes

Also allows fully setting the configuration of an OIDC provider from environment variables (for Google and Microsoft; additional values can be added later as necessary). OIDC providers that are set entirely in the environment variables will not show up on the frontend, as if they had "Social App Custom Data" defined. They can be accessed by visiting their hidden links:

/accounts/{Social provider ID}/login to login
/accounts/{Social provider ID}/login/?process=connect to setup

[bufke]

This seems pretty basic and it worked for me testing. Ok to merge once my comments are reviewed.