Bug 20756: OPAC "Share list" button should be styled with an icon

Bug 20756: OPAC "Share list" button should be styled with an icon

This patch adds a new "share" icon to the sprite image used to style
controls in the OPAC. The "share list" links now have a "sharelist"
class which is used to style the links with the new icon.

Also changed: Fixed the page structure markup of opac-shareshelf.tt.

To test you must have the OpacAllowSharingPrivateLists system preference
enabled.

 - Log in to the OPAC and go to Lists -> Your lists.
 - Create one or more private lists if necessary.
 - In the table of your lists there should be a "Share" link with the
   new icon.
 - View one of these lists and confirm that the "Share" link at the top
   of the table of list items.
 - Click through to the share list page and confirm that the page looks
   correct.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>