Sharing: update function to properly handle the sharing module

[jeherve]

1. Concatenate the frontend sharing stylesheet
   • sharing is the stylesheet added to the sharing settings screen
   • sharedaddy is added to the frontend
2. Add a conditional to handle disabled sharing css option
   • The option can be set under Settings > Sharing, and uses the sharedaddy_disable_resources option

cc @blobaugh and @MichaelArestad