Made the Gollum theme responsive

This isn't a major change in style or layout, just one that removes all the fixed sizing for browsers below 940px in width and ensures certain floating behaviours only kick in above 940px. This will ensure that future UI changes, such as changes to the editor that are being discussed in https://github.com/gollum/gollum/issues/826, will be easier to implement. It might also serve as a base for further improvements to Gollum's theme.