Docs here: https://github.com/sourcegraph/docs/pull/561
This PR adds support for using Bitbucket Server OAuth2 application links
for sign-in and permission syncing.
When used for permission syncing, the user's oauth token is used to
fetch user permissions (and now permissions are fetched via the server).
## Test plan
Tests added and updated.
## Changelog
- Sourcegraph now supports Bitbucket Server OAuth2 application links for
user sign-in and permission syncing.