[FIX] portal: use email_split method to avoid blocking valid emails, rather than obsolete email_re
[FIX] mail: mail_thread: fixed a bug in subscription, new followers
could overwrite the subscription of previous followers.

This has already been fixed in saas-2 branch at revision 9079 (revid
tde@openerp.com-20140109131748-hgtflp40pmjhlhu2). But the fix is also
valid for the 7.0 stable branch.