Instrument the SignupExpCTAButtonClick schema

Also fix a bug in #handleNoThanksForVisualEditor where a second
SignupExpPageLinkClick event would be logged without the user
clicking Edit or Edit source.

Change-Id: I6a2b1fdfde1034daa5ee99e136a054a6af329422
Merge "Instrument the SignupExpCTAButtonClick schema"