Userlogin.php: Tweaks and corrections

Userlogin.php: Tweaks and corrections

* Correct nesting of the block generating "Forgot your password?" link
  – it would be mistakenly only generated if the user could create an
  account.
* Use ->escaped() rather than ->parse() for 'userlogin-resetpassword-link',
  as it is used inside a link and allowing wikitext could lead to
  undesirable side-effects.
* Remove unused $extraAttrs variable.
* Remove pointless 'size' => '20' for some fields (they are displayed
  full-width using CSS).
* Remove pointless .mw-ui-block class, as its styles are always used
  for buttons in a VForm.
* Prettify code for hidden fields generation.

Change-Id: If749620bf2eaf58f815cae559383e1a1c842b870