Expand expedite label to revert MRs

What does this MR do and why?

Step 2 of gitlab-org/ruby/gems/gitlab-dangerfiles#28 (comment 1209372811).

• Rename expedite_master_broken_labels_* to expedite_labels_*
• Add support for revert MRs and pipeline:expedite label in Processor

From Draft to Ready

• Finish Step 1 of gitlab-org/ruby/gems/gitlab-dangerfiles#28 (comment 1209372811)

Expected impact & dry-runs

We'll be able to set pipeline:expedite on an MR whose title starts with Revert or revert 🎉

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review