Add tooling to handle PR reverts

[seanlip]

Is your feature request related to a problem? Please describe.

When a PR is reverted, this should be reflected clearly in the commit history, associated issues, etc. It would be great if Oppiabot could ensure this.

Describe the solution you'd like

When a revert PR is created, Oppiabot should ensure (to the point of blocking submission if the PR doesn't satisfy these guidelines):

• The title of the revert PR clearly indicates which PR it's reverting, e.g. it starts with "Revert PR #XXX: ...".

When that PR is merged:

• Oppiabot should apply a "REVERTED" label to the PR that was reverted.
• Oppiabot should leave a comment on the reverted PR saying that it was reverted, and pointing to the PR that reverted it. The comment should @-mention the author of the reverted PR.
• Oppiabot should change the title of the reverted PR to "[REVERTED] ..." (if the title doesn't already start with the string "[REVERTED] ".

Describe alternatives you've considered
N/A

Additional context
N/A