Make better use of scopes in the Workload presenter

Added a few more scopes to the Submission model in order to give the API there a more fluid feel and to make use of our own messages. Once those new scopes were in place, it was obvious there was another concept that was begging for a name there, so I did that as well.

I'm not entirely happy with the name `unmuted_submissions` here, mostly because that's not all the method is doing, but since each instance Workload presenter only deals with a particular language track, I decided to let that go for now.