core: replace regexp validation for user email.

validating using an ad-hock regexp may cause problems(see bug).
using strict java.mail.internetAddress() [1] for validation.
This enforces some (but not all) RFC822 syntax.

Using the strict form does not allow simple names (e.g 'user') but
requires a domain. 'user@localhost' is now accepted.

[1] https://javamail.java.net/nonav/docs/api/

Change-Id: I18904a669453baef632af1cd427b61852e7ab583
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1109169
Signed-off-by: Mooli Tayer <mtayer@redhat.com>