authz: Bitbucket Server ACLs documentation

Created by: tsenart

This PR adds documentation of, and a few code changes to, the Bitbucket Server ACLs feature, including the CHANGELOG.

Test plan:

I'd like to have reviewers follow the documentation that is added in this PR.

1. Check-out this branch locally and run ./enterprise/dev/start.sh.
2. Follow the docs to setup Bitbucket Server authorization.
3. Validate that it works as expected (i.e. Sourcegraph enforces permissions defined in Bitbucket Server)