Calibration moved to its own phase

Calibration is no longer an evaluation plugin, it is its own subplugin
and phase. This allows for greater flexibility and more clarity for
users using calibration in their workshops, as calibration results are
now available immediately for students. This will also enable more
functionality for Calibration in the future.
Add calibration subplugin

Whoops, this should have gone in the last commit :P
Assessment flagging

This update allows certain assessments to be flagged for manual review.
Flagging occurs in two ways: first, assessments whose grade is more
than 2 standard deviations from the median are automatically flagged in
red. Second, students can manually flag assessments they feel were
unfair, and these flags show up in orange. Students can only flag an
assessment once; after the flag has been resolved by a teacher, it
cannot be flagged again.
Bugfix: allocation upload

Allocations where the student has completed the assessment are no
longer deleted when reuploading allocations.
Fix confusing UX for no examples reassessment

Fixes a UX issue where a user could hit a dead-end when reassessment of
examples is disabled.