Properly implement 'system' field

This means the API now works by requiring users to query URIs in the
style of /sheets/<system>/<sheet_id>. The frontend hasn't been
adapted yet.
Add system URI parameter to frontend code

The frontend now works again with the new URI scheme.
The sheet route now implements its own serialize method
(as to why it has to, see this [1] issue), and everything
seems to work the way it should.

[1] - https://github.com/emberjs/ember.js/issues/1997