Bug 23791: Allow granular control of social networks enabled by SocialNetworks syspref

Bug 23791: Allow granular control of social networks enabled by SocialNetworks syspref

This patch updates the SocialNetworks system preference so that it
offers a choice of social networks (and email) which can be enabled in
the OPAC.

This patch also replaces the social network images with Font Awesome
icons styled so that the colors match the social networks' current
branding. The old social networks sprite image is removed.

To test, apply the patch and run the database update. Rebuild the OPAC
CSS:
https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client

 - Go to Administration -> Global system preferences -> OPAC -> Features
   and find the SocialNetworks preference.
   - If your SocialNetworks preference was enabled, all choices should
     be selected. Otherwise none should be selected.
   - If none are selected, select them all and save.
 - Locate a record in the OPAC and view the details page.
 - The social network icons should appear as expected and function as
   before.
 - Return to system preferences in the staff client and try enabling and
   disabling different options, verifying each time that the OPAC
   reflects your choices.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>