Due to the way review editing was implemented, we were losing comments on previous review edits.

Here, we add a new review.edit action that is specifically for editing, and make the existing review action fail silently (for backwards compatibility) in case it is used when there is already a review.

We also simplify the review data structure by only keeping track of one review per author per revision, instead of all edits. Later, if needed, it will be possible to keep track of all review edits.

Address @fintan’s feedback.