Use font awesome for icons

[mjtamlyn]

In particular the delete icon

[mjtamlyn]

@davidjrichardson this is an issue I'd love to see tidied up - the delete/edit links on the entry list page are a prime example. Similarly the X on the target list.

We will need to bundle the dependencies (as we do with all static stuff) not hotlink them to CDNs because the site is run often in offline mode at shoots. We may wish to look at a setting to swap this kind of thing out in production and use CDNs for performance reasons.