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
(cherry picked from commit 2db7b2605c2c1aae8d8cc3361e7a710a840deedb)
Merge "Blank some new Create account messages by default" into wmf/1.22wmf4