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