Make it possible for class teachers to view student list

[einarpersson]

It would be great if class teachers could sign in and view their students.

Option 1: Add a /teacher route
Option B: Reuse and expand /leader route to both show lists of students for activity and class, if leader email is assigned to them

Also, this would require that the admin create class page allows adding class teacher email(s).

[einarpersson]

After discussion with teachers this was decided to not be very important, and therefore not prioritized

[vilhelmprytz]

The current activity leader is actually a rewrite of the initial mentor interface. See https://github.com/tullinge/booking/pull/55/files