viewer: move option creation to main() to help with tests

queue: turn HTTPErrors into JSON responses via BaseHookHandler.write_error()

queue: log event type that hook handlers receive

queue: handle invalid JSON payload by raising HTTPError(400)

queue: log payload as the original JSON string to ease testing

queue: handle ping event

queue: add preliminary GithubHookHandler

queue: spell out hooks flavor

viewer: basic pagination on project page (to see all previous checks)

viewer: use SelectQuery.first() to get the last check or None