ProjectMirror: new helper class
Preferred to call them ProjectMirror rather than the context-lacking RemoteMirror. We should do something a like for the more feature-full Merge Requests. Amazingly, there was not direct GET for a given mirror, so we're resorting to listing all of them. We could later on add more interesting attributes, but this isn't what we are doing for other objects (yet). Here's a poor man advantage: by calling `api_get()` explicitely, there's no ambiguity between local (maybe stale) state and the server state.