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.
Some context: this uses an image (the icon svg) as a mask, to cut out a grey image in the shape of the icon.

Many browsers (not IE) support `mask`ing, but only webkit supports a mask with a size.

Do you think we should try embedding the svg and using a custom style sheet?