This change adds a new integration for linking to Trello cards from
review requests. This consists of a few pieces: the integration config
which connects to a Trello via a developer key, and a review request
field which allows people to search for and link to multiple cards.
The UI is currently fully functional, but at a later date I'd like to
make it look a bit more like the related object selector used in the
admin UI, with the search box shown above the list of selected items
(instead of swapping out the entire UI for what looks more like "tags"
inside the selectize widget).