Horizon icons should be abstracted through the Theme

Horizon is currently using Font Awesome as the provider for its high definition Icons. However, customization of specific icons should live within the theme itself.

It would be beneficial to use an abstraction layer between Horizon and the icons used rather than hardcoding Font Awesome directly in the HTML. Although we can simply override the Font Awesome styles directly, that creates a confusing layer of indirection when attempting to read or debug the compiled template html.

It is also noted that references to Glyphicons (the last default icon font used) still exist that need to be cleaned up.

Also, the loading of Font Awesome should take place through the theme itself.