Allow viewing source/diffs without width restrictions (BB-8405)

When viewing a changeset (pull request, or a commit, so such) the UI has a fixed width of 980px. This is really quite ridiculous on a wide screen with lots of whitespace on either side and the scollbar at the very bottom, making for a very poor user experience. When doing code reviews I usually open firebug and adjust the width to 95% and hey presto, nice wide layout and things scale nicely. Can we make that a permanent change and do away with the fixed width? (setting some sort of minimal width of course).