Feature/media services implementation

Implement MediaService using REST and XMLRPC for getMedia and uploadMedia.

This allows featured images to be set using the REST API and also to retrieve them one by one instead of depending of the sync of the full media library.

It will solve some reported bugs for setting the featured image.

cc @koke 
For consistency, I don't think we should search locally. Looking at `PostService` and others, `find*` methods search the local database and `get*` methods request objects from the network. I'd expect to be able to call getMediaWithID to force-reload a media object even if it's downloaded.

There's a similar use case in #2578.