mediawiki.ui: checkbox: Fix states according to spec

 * Color changes
 * Introduces hover state
 * Separates focus and active state
 * Removes need for inset border
 * Adds white icon for disabled & checked state

Design specification on Trello-
https://trello.com/c/JETLmm7F/7-check-boxes

Change-Id: I891f05c8edd010b81bd5f35eeae5d5dd22169933
Merge "mediawiki.ui: checkbox: Fix states according to spec"