mediawiki.less: Use less mixin to reduce code repetition in animation

Adds an `animation` and `transform-rotate` mixin to help with the animation.
Also creates a mixin for the keyframes and uses them for webkit, moz and
opera specific declerations.

Change-Id: I167dc5e5a2f43f64c5029864e76d42fcc82af622
Merge "mediawiki.less: Use less mixin to reduce code repetition in animation"