Made the Gollum theme responsive.

Not a particularly comprehensive change in style, just one that removes all the fixed sizing for browsers below 940px in width.

Closes #831.
Horizontal scroll when table is too wide
Fix css selector for Asciidoctor h1 html ouptput
Merge pull request #873 from Mogztter/issue-871

Fix css selector for Asciidoctor h1 html ouptput
Merge pull request #872 from Mogztter/issue-870

Resolves #870.
allow uploading files by drag and drop