Extract EE specific files/lines for app/views/{email_rejection_mailer,notify}

We have the following files containing EE specific code. We should move them to ee/

• app/views/email_rejection_mailer/rejection.html.haml
• app/views/email_rejection_mailer/rejection.text.haml
• app/views/notify/new_merge_request_email.html.haml
• app/views/notify/new_merge_request_email.text.erb

Should be straightforward with render_if_exists.