Move the code that creates the Editbutton for math from core,
where it is removed in Ib17aa47c7d647e1ed293541adbdd1aa77e25e59e.
Imported button image from:
ssh://gerrit.wikimedia.org:29418/mediawiki/core.git/skins/common/button_math.png
We don't need to vary the icon per language, because there is not a
single language that has a non default icon.
Bug: 60471
Change-Id: I3d126c1c252c27fa65ab8fb42ffab66a2d99cfa8
