Refactor: Topic#invite_by_email

Extract Topic#email_already_existas_for? and Topic#grant_permission_to_user
Fix failing spec due to missing variable in extracted method
Refactor: Topic#invite_by_email

Extract Topic#email_already_existas_for? and Topic#grant_permission_to_user
Fix failing spec due to missing variable in extracted method
Merge branch 'refactor-topic-invite-by-email' of github.com:cored/discourse into refactor-topic-invite-by-email
Merge pull request #1495 from cored/refactor-topic-invite-by-email

Refactor: Topic model