Add a database review feedback processor

What does this MR do and why?

This adds a database review feedback processor that comments on merge requests that have been labeled with databasereviewed or databaseapproved in order to solicit feedback to improve the database review process.

Relates to gitlab-org/gitlab#388005 (closed)

Expected impact & dry-runs

Should react to merge_request.merge and merge_request.close and post one unique comment only if the merge request is labeled with databasereviewed or databaseapproved.

Dry-run not available as this does not modify a schedule or policy.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations => gitlab-com/www-gitlab-com!119228 (merged)
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group => @gitlab-org/database-team
  • Relevant Slack channels => #g_database
  • Engineering week-in-review