Followup to small SVG default size

Make sure svg drawings on block images have the default size as
the wiki default, no matter their original set size.

Also took the opportunity to clarify the synchronization method
for scalable and image type in ve.dm.MWImageNode and call it when
the image model is loaded to make sure the details are always
synchronized before edit.

Followup to I682d832e456a501836f33ed1dfc7dbd78a4edf53

Bug: 62893
Change-Id: Ic47016d171634c532092067957cc2c3d431e9746
Merge "Followup to small SVG default size"