Contains schemamigration!

This commit allows assistants to give feedback for submissions in a way
that does not override the feedback received from the grader (though that
is still possible to do, if desired).

Viewing submissions (whether in inspect mode or from students view) will
show two feedbacks (assistant and grader) if available, or notice that
there is not feedback available.

Schemamigration adds assistant_feedback field to submissions
CONTAINS SCHEMAMIGRATION

Adds the possibility for assistants to give feedback about a
submission, without overriding grader generated feedback (though
this is still possible, if desired).

When viewing submission (either from student's view or from
assistants inspect view) feedbacks from assistant and grader are
shown separately. If feedback of that type is not available, info
is shown.
Fixes stupid spelling problems.
...
" "
This branch should now satisfy the functional requirements for the
assistant feedback feature.

The assistant feedback is now always rendered before the grader
feedback. Initial grader and assistant feedback (if they exist)
is shown in assistants/teachers assess view.
Merge pull request #133 from LassiHaaranen/assistant_feedback

Assistants can now give feedback without overwriting the grades