When a draft carries the __bugs_migrated marker, publishing no longer

copies the bugs_closed string. Instead, the review request's bug

relations are set from the draft's and the review request is marked as

migrated.
Publishing records one change entry per affected tracker under

bugs:<id> keys, with a label snapshot so history renders even if the

tracker later goes away. The legacy bugs_closed entry is still written

when the default tracker's view changes, for rbtools and the API.
Publishing a string-driven draft over a migrated review request pops

the review request's marker, reverting it to string-driven behavior so

the fresh string is read.

Ran unit tests.