Sharing: update function to properly handle the sharing module

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