Remove `constructive` extension of button and remove it with more
generalized `.mw-ui-progressive` class and rely on progressive color.
Also make use of central mediawiki.UI's variable.less file.
Bug: T136209
Change-Id: I22f365419c41dd888824dbfeb4b3c995713ac5a5
I decided to fix them all at once since we're merging all of these
patches together anyway. I can pull changes into other patches if
absolutely necessary.
Also fixes the dialog open/close handlers for the canvas click event,
and leaves the reuse dialog open on next/prev.
Change-Id: Id1564425442aec72e5e41f2f80986d8a104dd92c