Add option to select cache refresh interval.

Add option to select cache refresh interval in advanced settings. Setting
applies to all galleries, but is enforced individually on each gallery. This
should help users who load multiple galleries on same page as even basic REST
calls to Flickr are now not made.

Don't load codemirror scripts on all wordpress admin pages.

Changes to support serving AFG updates directly from Github to avoid manually
uploading files on dropbox. This will come into effect from next version.