PolicyGroup emails should have email format validation only

they are allowed to be non unique and blank
Fix tests on policy team
modify frontend to cope with the lack of an email address
update policy teams controller to use the name instead of email
Only skip validatios for PolicyAdvisoryGroup not teams

Also added in some tests
Move validation down into policy team
Merge pull request #158 from alphagov/policy-group-email-changes

Policy group email changes