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/
Fix alignment and appearance of boxes in review requests.

We have a bunch of stacked boxes in our review request and diffviewer pages.
When we introduced the new look for diffs, it became apparent to me that we had
some alignment and bleeding issues in these pages:
- Diff boxes were 2px less wide than anything else.
- Diff boxes didn't have a shadow.
- The collapse buttons for reviews and change descriptions weren't vertically
  centered.
- Change descriptions have a kind of wacky structure where there are two
  different elements with the "changedesc" class. This meant that we had some
  bleeding of the background color outside of the border (which I only
  noticed when zoomed way in).

Reviewed at https://reviews.reviewboard.org/r/4364/
Add a warning icon.

This uses the standard triangle and "!". There's a standard version
and Retina version.