Load media on a ruleset basis

Currently all media in Ember is loaded up front. All media is also specific to the Mason world.

We want to change this, so that only a basic set of media should be loaded at startup, and the full set of media should then be loaded when the user connects to a server. This would allow us to have different media per ruleset.
This feature would probably in some way work together with ember-dynamic-media-download blueprint.