option to enforce oauth for some users

Fix overwrite-groups config

This is a new implementation of the group handling introduced in #125

The previous implementation failed to preserve provider groups
properly. This also adds tests for t

Fix overwrite-groups config

This is a new implementation of the group handling introduced in #125

The previous implementation failed to preserve provider groups
properly. This also adds tests for the behaviour.

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Fix check of valid email domains

improved user validation, added tests

test cleanup

Allow mailRestriction to multiple domains

initial wizard generated checkin