In addition to adding to the help text a link to the user profile for a way to disable CodeMirror, this PR also improves keyboard accessibility by allowing the user to hit Esc to escape out of the CodeMirror tab trap. Note the changed help text:

CSS allows you to customize the appearance and layout of your site with code. Separate CSS is saved for each of your themes. In the editing area the Tab key enters a tab character. To move keyboard focus to another element, press the Esc key followed by the Tab key for the next element or Shift+Tab key for the previous element. You can disable the code syntax highlighter in your user profile. This will allow you to work in plain text mode.

Fixes #29.