Migrate to Pac4j framework for OAuth handling

Pac4j is now used instead of the no longer maintained predecessor scribe-up. This migration fixes problems with OAuth caused by the use of deprecated (Google) or discontinued (Facebook) third-party API versions.

added 4 commits

• a2ee8194...150cab8c - 2 commits from branch 2.x
• af09579e - Migrate to Pac4j for OAuth handling
• c2183aba - Set a default URL for redirection after OAuth authentication

Compare with previous version

enabled an automatic merge when the pipeline for c2183aba succeeds

mentioned in commit 328933d5

merged