Add the ability to apply labels to a changeset template

Created by: rrhyne

A user requested the ability to add labels to changeset templates so that they can track changesets made with campaigns.

We keep track of how all of our services are doing in terms of merging in PRs from our infra team and we use hacks like checking for “This pull request was created by a Sourcegraph campaign” in the PR body right now. Automatic GH labels (either configured in yaml spec or, even better, account wide and automatic) would be a lifesaver.

This feature would also help users leverage various workflows that rely on labels such as our tracking issues.