This change adds a sync_legacy_bug_list() method, which migrates the

legacy bugs_closed field of a review request or draft (default tracker

plus unattributed bugs) to the new Bug model. New IDs are attributed to

the repository's default tracker, or the sentinel without one. Stale

links in the bugs field are removed. The row is marked as migrated, with

the caller expected to save the extra_data field.
New drafts now copy the bugs relation from the review request, so the

draft starts from the current set of links.

Ran unit tests.