Adds a new page for Site admins to manage the list of sites for which
iframes are allowed by htmlpurifier.  Whenever an item is added,
edited, or deleted, the regex used by HTMLPurifier is updated.  Sites
are identified by favicon, and by a string entered by the Admin to be
used as the alt/title text for the favicon image.  The source of the
favicon image can be modified in config.php, but the google service is
used by default.

Change-Id: I4117de82691a002bf250ea71622eccfad4d5f8df
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>