Upgrade Bitbucket API to 2.0

This resolves #1945. From what I can tell the [OAuth API](https://confluence.atlassian.com/display/BITBUCKET/OAuth+on+Bitbucket) is still only under the 1.0 endpoint ([s/1.0/2.0 is 404](https://bitbucket.org/api/2.0/oauth/request_token)), so I haven't touched that part. Remaining items:

 - [ ] Remove Bitbucket elsewheres for accounts that have another sign-in option.
 - [ ] Update the `user_id` column with the new `uuid`s (we'll have to fetch all of those first).
 - [ ] Make sure Bitbucket team pages still work.