Code review comment for lp:~thumper/launchpad/fix-date-review-requested

Followed up on the outage last night for the Bazaar team. The active reviews page always expected the date_review_requested to be non-None.

However there are some edge cases where the proposal may be Approved but not reviewed. In this case it makes sense that the date_review_requested is None. The browser class has been updated to effectively coalesce the sort order to choose date_review_requested, and if that is None, use date_reviewed.

Also fixes the model level code where setting a branch to work in progress doesn't reset the date_review_requested so as to protect against accidental setting.

Tests:
  TestBranchMergeProposalListingItem
  ActiveReviewSortingTest
  TestBranchMergeProposalRequestReview