Update both Template Pack registration forms to use signup fields

• Nouveau and Legacy are now usinf the bp_xprofile_signup_args() to build the argument of their registration's form xProfile loop.
• Include a backward compatibility mechanism if the register.php page has been overriden from a theme or a plugin.
• This commit also includes PHP Unit tests about the xProfile loop and Signup Fields.

Props johnjamesjacoby, boonebgorges, DJPaul, Offereins

Fixes #6347