Added team invitations

src/pretix/control/templates/pretixcontrol/email/invitation.txt

@@ -0,0 +1,16 @@
+{% load i18n %}{% blocktrans with url=url|safe %}Hello,
+
+you have been invited to the team of an event that uses pretix for their
+ticket sales.
+
+Event: {{ event }}
+
+If you want to join that team, just click on the following link:
+{{ url }}
+
+If you do not want to join, you can safely ignore or delete this email.
+
+Best regards,
+
+Your pretix team
+{% endblocktrans %}