Bug 23903: Replace OPAC icons with Font Awesome

Bug 23903: Replace OPAC icons with Font Awesome

This patch replaces CSS-based background image icons with Font Awesome
icons. This will eliminate the use of sprite.png altogether.

To test, apply the patch and regenerate the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Because JavaScript events are often triggered by class, the changes made
in this patch require testing of JavaScript-driven behavior as well as
visual confirmation of the changes.

Enable OPAC features to expose them for testing: Login, Holds, Article
requests, Tags, Cart, Search term highlighting.

 - Perform a search in the OPAC
   - In the header at the top of the search results, check the
     'Unhighlight/Highlight' link.
   - Select some search results and verify that the multi-item controls
     are enabled: Add to cart/Lists; Place hold; Tag. Check that these
     controls work correctly.
   - Verify that the following links look correct and work correctly:
     'Place hold,' 'Request article,' 'Log in to add tags,' 'Add to
     cart.'
   - Log in to the OPAC and return to the search results page. Check the
     'Add tag' and 'Save to lists' links.

 - View the lists page
   - Test the 'Edit' and 'Delete' controls.
   - View the contents of a list. Test the following:
      - Controls at the top of the table of results ('New list,'
        'Download list,' 'Send list,' 'Print list,' 'Edit list,' and
        'Delete list').
      - Multi-selection operations: 'Place hold,' 'Tag,' and 'Remove
        from list.'
      - Controls for each item on the list: 'Place hold,' 'Add tag,'
        'Save to another list,' 'Remove from this list,' and 'Add to
        cart.'

   - View the bibliographic detail page for any search result
     - In the right-hand sidebar menu, verify that these controls look
       correct and work correctly: 'Place hold,' 'Print,' 'Request
       article,' 'Save to your lists,' 'Add to your cart,'
      'Unhighlight,' 'Send to device,' 'Save record,' and 'More
      searches.'
     - Test the shelf browser by clicking 'Browse shelf' under the call
       number for any of the holdings.
       - Test the 'Next' and 'Previous' controls.
     - Check the tabs for switching between 'Normal,' 'MARC,' and 'ISBD'
       views and verify that they look correct on each of those pages.

 - Add some items to the Cart and open the Cart
   - Test the cart controls at the top: 'More details/Brief display,'
     'Send,' 'Download,' 'Empty and close,' 'Hide window,' 'Print.'
   - Test the multiple-selection controls: 'Remove,' 'Add to list,'
     'Place hold,' and 'Tag.'

 - Log in to the OPAC as a user who has items checked out which can be
   renewed. On the 'Your summary' page, on the 'Checked out' tab, test
   the 'Renew selected' and 'Renew all' links.

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>