Revert buggy MediaWiki UI change and dependent changes

Reverts:

* c0ec2d64c0ea79e840072627407aeb16131e0f88
** mw-ui-button: Revamp the LESS to be simpler, and improve cross-browser support
[This breaks buttons in IE6-8 which previously were styled correctly and without problems.
This commit lacked rationale]

Unfortunately, this also means reverting these two commits which were
based off this one:
* b8a15fb6282cd329faf7fee359edcb530061b949
** Override vector anchor hover and visited styles on mw-ui-button
* 645ba9f1d229fa50572ec58d3932059281b84aac
** Remove mw-ui-button min sizing for inline mode

Change-Id: If5e55916795060fb3baadb75f997446527cbc27d
Merge "Revert buggy MediaWiki UI change and dependent changes"