Story 1163: Mobile Captcha support for editing

Add support for FancyCaptcha from ConfirmEdit extension. Reuses captcha
styling from account creation (CSS cleanup in the following commit).

Bug: 52047
Change-Id: I4850ed8eade5e76d141155d40f7ccd83d9cdfa57
Merge "Story 1163: Mobile Captcha support for editing"