Add feature flag to turn off requirement for team invitations

Fixes #1804

endpoints/api/team.py

@@ -41,8 +41,9 @@ def try_accept_invite(code, user):
 
 
 def handle_addinvite_team(inviter, team, user=None, email=None):
+  requires_invite = features.MAILING and features.REQUIRE_TEAM_INVITE
   invite = model.team.add_or_invite_to_team(inviter, team, user, email,
-                                            requires_invite=features.MAILING)
+                                            requires_invite=requires_invite)
   if not invite:
     # User was added to the team directly.
     return