This is largely a rewrite of the Bitbucket hosting service backend that

aims to throw out old Bitbucket API 1.0 conventions (such as error

introspection and pagination workarounds) in favor of API 2.0, and to

leverage the modern hosting service support in Review Board 4.0.
Much like the recent RB Gateway rewrite, the new BitbucketClient class

takes care of all communication, standardizing all error handling, URL

building, and authentication. It implements API accessors for everything

that the service backend needs, and it all goes through the modern HTTP

request code.
API accessors that deal with paginated payloads now wrap the results

using the new BitbucketAPIPaginator class, which gives us a single

standard way to handle pagination.
The service backend no longer directly performs any HTTP requests,

instead calling into the API accessors on the client, providing

arguments based on the account or repository information and turning the

results into the proper objects (or returning them directly).
The truncated commits handling for the WebHook push event also leverages

the new paginator class, using it to limit the number of pages of

results and to simplify the logic, making it less error-prone.
The authentication form now rejects Bitbucket usernames comprised of

e-mail addresses, which has been a common source of confusion ever since

Atlassian changed to having users log in via their e-mail accounts but

kept usernames for the Bitbucket API.
Documentation has been added all throughout, and unit tests have been

updated for the new requirements.

Linked an account with Bitbucket successfully (and verified that it

provides a useful error when trying to do this with an e-mail address).
Configured a repository. Verified it found the correct repository data

from the API and set everything up.
Browsed for commits. Saw it populate the list of branches and the list

of commits correctly.
Posted a commit for review. Saw it extracted the information and diff

from the commit correctly.
Viewed the diff without any issues.
Unit tests pass.