Move Create account messages and links here.

Ie0888cd0 and I9e33c21a make the default createacct-helpusername and
createacct-imgcaptcha-help messages blank.  Here we provide WMF-specific
defaults for these messages and links within them.

Bug: 47801
Change-Id: I28b0079b971e5f6cf5196d5581bbfd7a3ac405f9