Captcha.php: modified to add captcha to 'extrafields' in Userlogin and Usercreate

Captcha.php inserts captcha in the header of Userlogin and Usercreate template.
This leads to inconsistent view because js,if enabled, moves the captcha before
submit button. Now the captcha is inserted at the 'extrafields' parameter just
before submit button in both the templates,for uniformity of view.

JS functionality in
resources/src/mediawiki.special/mediawiki.special.userlogin.common.js migrated
to backed in FancyCaptcha.class.php, so that the FancyCaptch is already styled
and positioned before 'submit'.
Depends on I82c68814e79cbc5aa250a308862c59fcbb6fd527
Depends on Ie95305b2e6dcbf527a23c92613755092185e6a05

Bug: T85192
Bug: T87190
Change-Id: If9a68aaee2cf98d63647816ccc8fc0bad12ca3d3