[frontend/user management] Filtering Users

[javawolfpack]

Describe the solution you'd like It would be nice to be able to search or filter the User Administration page. If there are 100s of users and need to find/delete just one of them currently requires manually searching numerous pages which is not feasible.

This is a side effect of the fact usernames are not auto-populated via SAML mentioned in bug https://github.com/UCL-INGI/INGInious/issues/953 so making sure students use the desired username is impossible as they have no way to edit after creation.