Resolve "Manage staff/Assign TA to cluster(s) page"

This MR adds a Manage Staff page where there is:

• Functionality to assign TAs to clusters and/or repos in gitlab
• Functionality to unassign TAs from clusters and/or repos in gitlab
• Functionality to view which TAs are in which clusters in a nice way (cross checked with gitlab/core)
• Functionality to add TAs/Head TAs to course edition (only TEACHER role not TEACHER_RO)
• GraphQL API request to efficiently get all groups and all projects "recursively" in a gitlab group (edition) -> needed when removing staff from edition since it removes them automatically from all those.

Some pictures below for demonstration:

Depends on !9 (merged) !!!!

Closes #23 (closed)