Fix a padding issue in the administration UI.

The top of every content area in the administration UI had excess
padding, due to the addition of padding-bottom on the 'navbar-container'
element. Non-admin pages were updated for this, but the admin pages were
not. This just removes the content area's padding-top on these pages.

Reviewed at https://reviews.reviewboard.org/r/5922/
Changed Content-Disposition from inline to attachment for raw diff download.

When chrome is presented a file with Content-Disposition: inline, it shows the
file in the browser. This is fine for most things, but for the "Download Diff"
link, we'd prefer to offer to download the file.

Testing Done:
Manual testing with Chrome Version 35.0.1916.114, the affected browser. Also
tested Firefox 29.0. In Chrome, Download Diff now downloads instead of
displaying in the browser window. Added unit test.

Fixes bug 3384.

Reviewed at https://reviews.reviewboard.org/r/5893/
Merge branch 'release-2.0.x'