CSS reorg: concatenate the frontend sharing stylesheet

'sharing' is the stylesheet added to the sharing settings screen
'sharedaddy' is added to the frontend
CSS reorg: add a conditional to handle disabled sharing css option

The option can be set under Settings > Sharing
uses the sharedaddy_disable_resources option
Merge pull request #997 from jeherve/css-reorg

Sharing: update function to properly handle the sharing module