Template 'jwinf' – Admin interface: Change text for deletion in admin group interface for JwInf 2022

Template 'jwinf' – group page: Change delete button color, modify texts in preparation to JwInf 2022

This reverts commit 59ac0842.

This also fixes the login box being shown in the new jwinf-template admin interface for logged-in users.

Template 'jwinf': Allow teachers to directly access admin user page for student deletion. Closes #138

Tests: Add tests to check that teachers can still compete after a contest's time is up while students can not.

Allow teachers to participate in timelimited contests for unlimited time. Also simplifies time limit checking. Closes #140

Fail harder during contest reading in order to not silently ignore invalid contest files. Fixes #135.