Port mediawiki.ui SASS to LESS

This was the only SASS in core.

Remove gradient hacks for IE<10 (proprietary filter property) in
vertical-gradient mixin. Buttons look fine without gradients too,
and filter can often signfificantly degrade performance:
http://gomilkyway.com/web/why-site-is-so-slow-on-ie-even-to-scroll-poor-ie-performance/
http://coding.smashingmagazine.com/2010/04/28/css3-solutions-for-internet-explorer/
http://msdn.microsoft.com/en-us/library/ms532847%28VS.85%29.aspx
(look for "performance")

Remove text-shadow for button's text which had no effect on appearance.

Remove obsolete or non-existent vendor-prefixed properties:
* display: -moz-inline-stack (not needed for Firefox 3+)
* -webkit-border-radius, -moz-border-radius (supported for sometime
  without a prefix, graceful degradation for old browsers)
* -ms-border-radius, -o-border-radius (never existed)

Remove *vertical-align: auto from IE6/IE7 inline hack. Can't find any
evidence that it's necessary.

Bug: 54485
Change-Id: I81d8d0c9f6705aef8bdb8a28e1154f2de4301235
Merge "Port mediawiki.ui SASS to LESS"