This change adds the new review request field(s) for bugs. The old

static BugsField is replaced with dynamic TrackedBugsField instances

when trackers are configured.
Each configured tracker that applies to the given review request appears

as a separate field. These are either editable or read-only depending on

whether the acting user passes the conditions in the tracker.
The field for the default tracker reads through get_bug_list(), so it

also covers unattributed bugs and unmigrated legacy bugs_closed data.

Ran unit tests.
Configured multiple trackers and saw that they appeared correctly.