Add the loading throbbers to the Seven style guide

Problem/Motivation

The current components for AJAX loading affordances are inconsistent, fragile, and difficult to override. If we include these components in the style guide it gives us an opportunity to improve consistentcy, improve themer experience, and document the choices and when to use them.

Related issues

• #1986434: New visual style for Seven
• #2032773: Use Libricons (icon font) in Seven, consider using it more broadly in core
• #1847916: Replace the ajax-progress-throbber div with a class
• #1974928: Update throbber icon

Resources and inspiration

• Ladda
• http://cssload.net/
• http://www.seebeetee.com/2013/08/29-great-loading-animation-from-codepen.html
• http://codepen.io/collection/HtAne