Templates:Captcha position modified,extend functionality added to QuickTemplate

Templates:Captcha position modified,extend functionality added to QuickTemplate

The ConfirmEdit extension presently inserts captcha in the header of Usercreate
and Userlogin templates. The js,if enabled places the captcha before submit
button.  A separate 'extrafields' placeholder for captcha is added just before
submit button in Usercreate.php so that captcha position remains consistent,
even if js is disabled.  An extend method added to QuickTemplate to append any
data to existing field, rather than setting it with fresh data. Particular use
for 'extrafields' in Usercreate and Userlogin.

Bug: T85192
Change-Id: I82c68814e79cbc5aa250a308862c59fcbb6fd527