Improvements to rendering of long path names in commit view

I noticed that in Firefox the diffs were completely unreadable for files with path names that took more than one line to display. For Firefox, the change is:

1) Remove float:left from the meta class. This is the main cause of the problem. However doing this caused other problems because the floating buttons to the right were shifted down.

2) Put the buttons first in commit.twig. This means the filename text can flow around the buttons again.

3) Remove the fixed 28 pixel height limit from the source-header class as the height depends on how many lines there are.

This fixed Firefox, which was the worst affected. However I noticed that only Firefox wraps text on slash characters, so the fix had little effect on other browsers. The second commit adds a "filename" class which sets word-wrap:break-word. This slightly improves matters on Chromium and IE, which now break long filenames across lines in some circumstances.