Add a bugbot command for requesting webhooks

This is to address freedesktop#705 (closed) which requires three components:

• bugbot request-webhook to be called by the project maintainer, identical to damspam request-webhook
• bugbot process-webhook-request to be handled by a hookiedookie instance, identical to damspam process-webhook-request
• bugbot gitlab-triage with the actual invocation of gitlab-triage

Most of the work here is reshuffling and renaming things to drop the various damspam prefixes and type names, the bugbot command is added in one commit - the logic is just enough to warrant it here as opposed to having a shell script.

By default the command fetches .triage-policies.yml from the main or master branch (whichever exists first) and runs that against the given issue or mr (both specified with a url). The deployment in fdo-bots should make use of the --delay argument so users get a chance to unset a label if it was set accidentally.