Patch service links module to allow custom social sharing icons

[rooby]

This is a follow up to issue #290.

Currently with the service links module to specify an images directory in the theme you have to hard code the path, which is incompatible with the file system on a GovCMS site.

There is a patch for the service links module at https://www.drupal.org/node/2760649 to allow for tokens in this path, including a theme path token.

Based on history I don't expect that patch to get committed any time soon so we might have to use the patch in GovCMS.

Someone reviewing that patch would also be appreciated.