(bug 22457) More natural flow for the "By e-mail" feature

This feature is on Special:Userlogin/signup to allow a random new
password to be sent to expected account owner on assisted signup.

It uses JavaScript to hide the password & retype password fields when a
logged-in user creates an account (for someone else) and checks the by
e-mail feature.

Change-Id: I7bf57eef64fea19a8237f997019e0b2bfdf401b3
Merge "(bug 22457) More natural flow for the "By e-mail" feature"