The refactoring of views into mixins broke subtly broke the diff context

resource. It was passing in all arguments positionally and the new

CheckLocalSiteAccessViewMixin was pulling local_site_name out of both

kwargs and (mistakenly) from a caller passing *args. We now pass all

arguments in as keyword arguments to avoid this mess.
This patch also removes a bunch of dead imports I noticed in

reviews/views as a result of the aforementioned refactor.

Ran unit tests.
With this patch applied, there are no more 500s on the diff context

  resource when viewing interdiffs.