allow users to edit team member's work

if a team member has submitted work, in team mode, that is equivalent
to your own work, and you can edit it.
random examples filter by workshopid

In random examples, there was a bug where get_n_examples would return
examples from other workshop instances. This is now fixed with the
addition of a new field in workshop_user_examples.
Add continue button to examples review
Fixed bugs in calibration

This fixes two problems in calibration: if competent reviewers had
given all zeroes, it would falsely flag the submission as having no
competent reviewers; and if the random examples were being used, then
reviewers would get zero for their grading grade, because workshop
considered that they had not done enough examples.
Self allocation for team mode

Random examples in team mode now supports self review. Self review
means that the team submission is reviewed by all team members, not
just the original submitter.

This also fixes a spelling error and a moodle coding style error.