Remove inserted leading whitespace

This is bit of a hack, as leading whitespace could be
significant if styled with white-space:pre.

Long term VE shouldn't be editing the user's HTML, and
should just highlight potential formatting issues.

We avoid the stripping in preformatted elements as we
expect they will have that styling.

Bug: 51462
Change-Id: I654d98e17dd604cb2a192831ff3f3597f95b9962
Merge "Remove inserted leading whitespace"