Blank some new Create account messages by default

Do not show the "(help me choose)" and "Can't see the image?  _Request
an account_" links.  They can still be customized on-wiki, but the
default value is empty.

The content and URLs that we encourage for these messages on WMF wikis
are now in extension WikimediaMessages (change I28b0079b).

Add comment to vform create account JavaScript.

Bug: 47801
Change-Id: Ie0888cd0582dc3f63ae569097159a4d9b171a5df