Migrate to ueberauth_oidcc

[AbsurdlySuspicious]

Migrated custom OIDC auth provider to more modern ueberauth_oidcc

• Initally I've encountered same issue as in OIDC Login redirects to login page #469 and this change fixed the issue
  • This may also fix OIDC is crashing docker container #489 (?)
• This is also improves security as new implementation supports PKCE and id_token nonce

Note that:

• This introduces new env var OIDC_ISSUER as new implementation appends .well-known by itself
  • But my patch tries to be backwards compatible with previous OIDC_DISCOVERY_URI by stripping additional path
• New implementation has more strong validation, some of idP`s my stop working because of this

I have zero experience with Elixir so I apologize in advance if I made something wrong.
Also sorry that I didn't open issue beforehand, I've initially planned this PR as a fix for #489

[simonprev]

Nice thanks for the PR! We don’t use oidc internally so I can merge this and release to see if it fixes the issues.

[AbsurdlySuspicious]

Made all requested changes