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