Store next_url in the database during the OAuth2 authorization flow

[leafty]

GitHub wants an exact match between the configured redirect URL and the request parameter used during the authorization flow. This means that we should avoid sending a redirect URL which contains query parameters and instead store next_url in the database.

Example configured callback URL:

https://ci-renku-3620.dev.renku.ch/api/data/oauth2/callback?next_url=https%3A%2F%2Fci-renku-3620.dev.renku.ch%2Fv2%2Fconnected-services

Solving this may also make PKCE work (as we need to save the PKCE code as well). #193