Improved "Additional email addresses" support

We've had various problems with additional addresses lately, so here are some patches.

They do:
* Validate Additional addresses in email_form->validation(), so errors are thrown before others are sent.
* Improved error for invalid address.
* Add support for "Name <email@example.com>" format.
* Add help button that explains Additional addresses field.

Adding support for the <> format required moving additional_emails to PARAM_RAW, as tags are stripped from PARAM_TEXT. All locations where it may exit are cleaned with s(), validate_email(), or clean_param().