Switch from SVG to CSS animations for loading class

[jace]

In _layout.sass, we define a .loading class that uses an SVG <animateTransform> element. This causes the following warning in Chrome: SVG's SMIL animations (<animate>, <set>, etc.) are deprecated and will be removed. Please use CSS animations or Web animations instead.

Baseframe should remove the SVG animation and specify a CSS animation within the class.