Fix MWBlockImageNodes' default horizontal location code

Introduce method ve.ce.MWBlockImageNode.getCssClass and use it instead of
accessing ve.ce.MWBlockImageNode.static.cssClasses - it gives a better
way of handling special cases.

Bug: 51995
Change-Id: I236b08c8345a10d144f392ec37fd130cf5f7737d
Merge "Fix MWBlockImageNodes' default horizontal location code"