repo-updater: Exclude patterns for Bitbucket Server

Created by: keegancsmith

For some customers it is impractical to enumerate the valid repository patterns, since the list is large and constantly changing. However, they just want to exclude a specific project. This commit adds the more general ability to exclude based on a regular expression for Bitbucket Server. Its approach can be extended to other code hosts as needed.

Test plan: unit tests

Fixes https://github.com/sourcegraph/sourcegraph/issues/3558