Update throbber icon and use SVG

Problem/Motivation

The default ajax progress throbber icon looks poor on a retina display. You actually don't need retina to know this is true.

Proposed resolution

Re-design and replace the outdated GIF used in the background image of the throbber class with a scalable vector graphic.

Before:

After:

Remaining tasks

1. Test cross-browser support of <animateTransform>, to ensure we don't have any breakage across our supported platforms/devices.
2. Propose a throbber style, and settle on a choice. Alternatively, we could decide to use the visual style / look of the existing throbber.
3. Create an SVG based on ^ and animate it.
4. Create a patch.
5. Make the ajax progress SVG spin with CSS animations.
6. Test patch

User interface changes

The ajax throbber has a new look:

• Attach an image to an article node (/node/add/article).
  
• Enable or disable a view (/admin/structure/views) using the dropbutton.
  Collapsed
  

  Expanded
  

API changes

• There is a new "is-throbbing" class that adds the CSS3 rotation animation to the SVG.