regression: discussions list should not show repository except on explore page

Created by: slimsag

@francisschmaltz your PR https://github.com/sourcegraph/sourcegraph/pull/1824 affected two locations it shouldn't have:

(1) viewing only discussions in a file, it should not show the repository (as a user I already know I am in that repository):

(2) viewing discussions list on the repository overview page (again, as a user I know I am already in that repository):