Add icons to button links on plugin list pages and plugin detail page

Added icons to the button links on plugin pages for improved usability. (I think they were lost in the redesign).

I decided to swap out Glyphicons with [Font Awesome](http://fortawesome.github.io/Font-Awesome/) because these two don't play well together. Since it is an icon font, it will scale up nicely to retina displays and should improve page load performance slightly due to fewer image objects to download. I did check across the whole site to make sure that removing Glyphicons doesn't break anything. 

Also Font Awesome has a [much larger set of icons](http://fortawesome.github.io/Font-Awesome/icons/); if this change is accepted then I plan to take this further and replace all icons across the site where possible.

See screenshot:

![screenshot from my local](https://www.evernote.com/shard/s3/sh/21f7cc0e-10ae-4fad-856a-16917253828f/6ffbaa807468446fd335823f693376d1/deep/0/Screenshot%207/30/13%2010:41%20PM.jpg)