Convert Special:EmailUser to use OOUIHTMLForm

Convert Special:EmailUser to use OOUIHTMLForm

It already used HTMLForm for the main form, but the form, which asked for
the username didn't.

Converted the "Enter username" form to use HTMLForm and show it, if no user-
name was passed to the form, show the main "Send e-mail" form, instead.

Extra points:
 - Let HTMLForm::setSubmit*() function return it's own HTMLForm instance

This basically is a squashed version of these changes:
I6231577047c93781496e0f8af6809e2ef49e662a
I3e0c02155428ae400bc3a6d3ed2e66e69ee441fa

Bug: T117791
Change-Id: If8bf42b2bd092706f2e580083b2400121d35c41c