Added Submission#assignTo(email, expiry).
Added Submission#getAssignee().
increase timeout for webhook tests
Mentors must now assign submissions to themselves before assessing.

They have 15 minutes to assess a submission by default, at which point
someone else can assess it.

Note that this is not intended as a security mechanism, as it's technically
still possible for a properly-formed POST request to add a review to
the assessment; this is simply done as an aid to avoid race conditions.
refactored submission detail view a bit, added tests.