Restore chooseAttr.html

This file was deleted accidentally and is needed for exam creation.
Init exam subscription dates before starting for the first time

Many places now rely on the precence of dates for the subscription periods so
we now initialize these dates before showing the exam for the first time.
The default for the start of the subscription period is the following day, 00:00,
and for the other two dates "+ 1 month" 23:59.
Add link to "course" detail view into the toolbar in the exam view