* [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
* start moving some database stuff around
* continue moving db stuff around
* more fiddling
* more updates
* and some more
* and yet more
* i broke SOMETHING but what, it's a mystery
* tidy up
* vendor ttlcache
* use ttlcache
* fix up some tests
* rename some stuff
* little reminder
* some more updates
tobi