db: introduce constraint ensuring no duplicate (reviewer, pullrequest) combinations

A reviewer should only be added once to a review.

Previously, this was not ensured by the database itself, although that the
controller would try to not add duplicate reviewers. But there was no hard
guarantee: e.g. simultaneous adding of the same reviewer to the same review
by a review owner and admin, a framework bug that sends the same request
twice, ... could still trigger duplicate addition. Additionally, code
changes (e.g. a new API) could introduce bugs at the controller level.

Existing production databases were found to contain such duplicate entries.
Nevertheless, as the code displaying reviewers in a pull request filtered
out duplicates, this never showed in the UI, and never was a 'real' problem.

Add a UniqueConstraint in the database to prevent such entries, with a
database migration step that will first find and remove existing duplicates.