Display only visible repositories via API and Website, even when logged in as administrator.

Display all repositories only in admin section.

I propose this change because the Repository List API documentation says "This
will only list visible repositories. Any repository that the administrator has
hidden will be excluded from the list.". Currently, when accessing this API
whilst logged in as administrator this rule isn't met.

Testing done:
Tested manually on sample installation. Also tested with the unit tests.

Reviewed at https://reviews.reviewboard.org/r/3279/