[Feature]: improved admin interface

[Harniver]

Objective

Improve the admin interface so that it is effective to manage special cases and monitor the progress of a running contest.

Acceptance Criteria

How do we know this is done?

• show in the admin the total number of valid students (not absent/deleted)
• also show the subtotal of online students (with a userAgent) among them
• and also show the subsubtotal of students online right now (with an open window)
• for single-turn contests, add a checkbox to allow new turns to be made for a school
• show in the school detail table a link to access as that school teacher
• Tests updated/added

Notes & Approach