* [feature] overhaul the oidc system
this allows for more flexible username handling and prevents account
takeover using old email addresses
* [feature] add migration path for old OIDC users
* [feature] nicer error reporting for users
* [docs] document the new OIDC flow
* [fix] return early on oidc error
* [docs]: add comments on the finalization logic
Dominik Süß