Add HarbormasterHookController as an entry point for all Harbormaster hooks
Summary: This makes the set of hooks easily extensible, as a first step toward integrating more 3rd party CI in phorge.
Test Plan: Send requests to /harbormaster/hook/circleci/ and /harbormaster/hook/buildkite/ and check they run the proper handler.
Reviewers: O1 Blessed Committers, Matthew
Reviewed By: O1 Blessed Committers, Matthew
Subscribers: Matthew, speck, tobiaswiese
Maniphest Tasks: T15018
Differential Revision: https://we.phorge.it/D25005