File Manager UI enhancements.

- Add a 'Download' button to each tab
- Add a 'View' button to the Scripts tab
- Unify Download/Delete/View code into a single set of JavaScript
  and PHP functions rather than one function per action per file
  type.
- Replace real-ajax-uploader by jQuery-Upload-File.  There are some
  nicer jQuery upload plugins out there, but the changes for them
  would be more invasive and pull in a lot more files.  We can
  evaluate whether to switch to another solution at a later time.
  jQuery-Upload-File is covered by the MIT license.
- Include jQuery-Form-Plugin which is a dependency of
  jQuery-Upload-File.  jQuery-Form-Plugin is dual-licensed by the
  MIT and GPL licenses.  It is included here via the GPL license.
- Add some more helpers to config.php for getting a setting out
  of the settings array.

Fixes #78.