Attach default reviewers to bitbucket server PRs

Created by: eseliger

Since we deemed this an oversight and not an additional feature, I went ahead and implemented it as a "bug fix". It wasn't much effort to do and works alright, so I guess we could merge it. Deferring to other engineers and product to make that decision.

Demo PR: https://bitbucket.sgdev.org/projects/SOUR/repos/automation-testing/pull-requests/142/overview (user thorsten is assigned)

Closes https://github.com/sourcegraph/sourcegraph/issues/20472 Closes https://github.com/sourcegraph/sourcegraph/issues/14746