Image insertion alignment fix

This removes the initial alignment (either left or right) from new
inserted images, letting the wiki decide the default positioning.
Also, it makes sure that VisualEditor positions the image properly
(according to wiki defaults tright/tleft for ltr/rtl) when editing.

Bug: 51851
Change-Id: I25b966cf6f2736437509ea7e70bfda1bdbc79021
Merge "Image insertion alignment fix"