Add font-awesome-sass gem

We will use this for icons
Rename _slide_items.html.slim

Use a more semantic name for this partial
Use fa-arrows-v for handle
Rename partial _group_items.html.slim

Use a more semantic _sortable_groups.html.slim name for the partial
Use font-awesome icons
Add padding to handle element

Gives the icons a little more room and makes the sortable items look nicer
Rename partial _overrides.html.slim

Use more semantic and consistent name
Use font-awesome icons
Use the button warning style for remove button
Merge branch 'font-awesome-sass'

We now have a good library for icons to use in ISK with font-awesome