[patch] Clean up bp_core_signup_send_validation_email()

[cnorris23]

The function, bp_core_signup_send_validation_email(), contains a lot of code from the function, bp_core_activation_signup_user_notification(), for WPMU. Some of this code is redundant. The $message_headers variable, for one, isn't needed, because wp_mail() gives the exact same results by default. The $from_name and $admin_email variables are already handled by the bp_core_email_from_name_filter() and bp_core_email_from_address_filter, respectively.

The filters for $to, $subject, and $message are named after the WPMU specific function.

It would also be nice to have some extra parameters for the $message filter so that the email can be more easily customized and personalized.

[DJPaul]

I want to go over emails in a big way, so if that doesn't happen, we'll put something like this on.

[DJPaul]

See also #2745

[boonebgorges]

(In [3623]) Cleans up bp_core_signup_send_validation_email(). Fixes #2404. Props cnorris23. Also fixes some PHP warnings.

[boonebgorges]

Paul - I know that email overhauls are in the pipeline, but I figured not until after 1.4. Thus [3623].