Issue icons

* Update issues fixture to reference new icon set
* Grey icons for featured projects on the home page via image-mask
* Macro to use a field's "url" property if present and an empty string otherwise

Both S3 and the shared media repository have been updated.
The use of the "-webkit" prefix here without a moz prefix or just a generic one seems to point to this property not being available in non-webkit browsers. Are we just accepting that for this iteration?