This change adds API resources for the new bug tracker features:
/api/bug-trackers/ and /api/bug-trackers/<id>/:
Read-only listings of the bug trackers usable by the requesting user.

Disabled trackers, the sentinel, and trackers whose user conditions the

requester fails are never listed. Configuration management stays in the

administration UI for this phase.
/api/bug-trackers/<id>/bugs/?q=&review-request=:
Typeahead search, implemented with BaseBugTracker.search_bugs(). This

will the per-integration ajax views that were implemented for Trello and

Asana. The endpoint requires a review request and verifies, server-side,

that the tracker applies to it and that the requester passes the

tracker's user conditions.

conditions.
A bugs field on the review request and draft resources:
Entries of {id, tracker, url?, summary?}, with url/summary omitted

when the requester fails the tracker's conditions. Draft writes take

comma-separated [<tracker-id>:]<bug-id> tokens (bare IDs go to the

default tracker, or stay unattributed). A write replaces the links on

every tracker the writer passes conditions for; links on trackers the

writer fails are preserved untouched, and naming such a tracker is an

error.
/api/review-requests/?bug=<id> and  /api/review-requests/?bug-tracker=<id>:
Filters on the review request list resource, joining against the bug

relations.

Ran unit tests.