Bug 22568: Replace RSS icon in the OPAC with Font Awesome

Bug 22568: Replace RSS icon in the OPAC with Font Awesome

This patch removes the RSS icon image (feed-icon-16x16.png) from the
OPAC and replaces it with Font Awesome icons styled to match.

To test, apply the patch and regenerate the OPAC CSS. View the
following pages, confirming that the RSS icon looks correct:

 - OPAC home page (RSS news icon)
 - Search results page (Subscribe to search)
 - Recent comments
 - Search history
 - Lists -> View list

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>