Switch email headers to an array.

The passed WP email headers can either be a string *or* an array.  For
consistency, convert email headers to an array to avoid any problems with
email plugins.
Brackets!
Fix notice.
Add user ID to legacy group forum listener.

This will allow us to potentially alter the "From" email header.
Whitespace.
Set the "From" name in RBE emails to the member's display name.

See #7.