Use bp_core_validate_email_address() at registration

[boonebgorges]

In r6279, bp_core_validate_email_address() was introduced to consolidate some of the disparate email checks done by WP and BP into a single, reusable function. For BP 1.7, we should ensure that this function is used wherever email validation is done in BP, in particular during the registration process, in order to reduce reduplication of code.

[boonebgorges]

(In [6336]) Use bp_core_validate_email_address() during registration

This centralizes email validation between registration and the Settings
component.

Also introduces bp_core_add_validation_errors(), a utility function for
converting the return value of validation functions into WP_Error-style
messages. This can be used in the future, if better username validation
functions are introduced.

Fixes #4486