Add a scope for filtering submissions by language
Add a scope for excluding submissions by user
Use the new Submission scopes
Remove duplication in use of scopes.

The only difference between the `pending` and the `needs_input` method
is the use of the scope for what state the submission is in. Since we're
dealing with unmuted submissions for a particular language, give that
concept a name via a method and use it to remove the common scopes used
in both the `pending` and `needs_input` methods.
Merge pull request #1725 from mattr-/more-scopes-for-submission

Make better use of scopes in the Workload presenter