User authentication using OAuth2

[se-bastiaan]

Closes #141

Summary

This switches our authentication method to using OAuth instead of the previously used "proprietary" token authentication. It also completely rewrites how network requests are executed.

How to test

Steps to test the changes you made:

1. Login using your local instance

• To do this you need to define the client id of your application
• For production and staging these are registered here (if they already exist):
  https://thalia.nu/admin/oauth2_provider/application/
  https://staging.thalia.nu/admin/oauth2_provider/application/
• This only works when you have this option available: Add OAUTH_SCHEME setting concrexit#1353

2. Check every place where we do network requests
3. All works!
4. Now revoke your access token here: https://thalia.nu/admin/oauth2_provider/accesstoken/
5. The app should now request a new access token using the refresh token
6. All still works!

Todo

• Fix tests
• Add documentation to getting started
• Fix CI builds

[se-bastiaan]

Needs GitHub Actions still

[KiOui]

Is this one ready for a review? Also for the concrexit side of things?

[se-bastiaan]

Yep

[se-bastiaan]

But maybe it would be nice if I add the github actions properly and then you can just download the IPA