Improved examiner-student organizing tools

[torgeirl]

When organizing examiners for an assignment, the UI allows you to filter the listed users by tag:

There is however no visual indication for which group examiners or students belong to when you bulk-organize your examiners (#1157). Ie. you can select all students with a tag and click 'Select all' for them to give the selection an examiner, but are then presented a list of all examiners without any indication which tag they have:

This could be solved either by re-working the UI to included indicators for group memberships, making another bulk option for linking students and examiners with matching group memberships, or by adding an group-matching option to assignment creation.