FIX: don't allow inviting more than max_allowed_message_recipients

* FIX: don't allow inviting more than `max_allowed_message_recipients` setting allows

* add specs for guardian

* user preferences for auto track shouldn't be applicable to PMs (it auto watches on visit)

Execlude PMs from "Automatically track topics I enter..." and "When I post in a topic, set that topic to..." user preferences

* groups take only 1 slot in PM

* just return if topic is a PM

spec/components/guardian_spec.rb

@@ -568,6 +568,7 @@ describe Guardian do
       let(:user) { Fabricate(:user, trust_level: TrustLevel[2]) }
       let!(:pm) { Fabricate(:private_message_topic, user: user) }
       let(:admin) { Fabricate(:admin) }
+      let(:moderator) { Fabricate(:moderator) }
 
       context "when private messages are disabled" do
         it "allows an admin to invite to the pm" do
@@ -586,6 +587,22 @@ describe Guardian do
           expect(Guardian.new(user).can_invite_to?(pm)).to be_falsey
         end
       end
+
+      context "when PM has receached the maximum number of recipients" do
+        before do
+          SiteSetting.max_allowed_message_recipients = 2
+        end
+
+        it "doesn't allow a regular user to invite" do
+          expect(Guardian.new(user).can_invite_to?(pm)).to be_falsey
+        end
+
+        it "allows staff to invite" do
+          expect(Guardian.new(admin).can_invite_to?(pm)).to be_truthy
+          pm.grant_permission_to_user(moderator.email)
+          expect(Guardian.new(moderator).can_invite_to?(pm)).to be_truthy
+        end
+      end
     end
   end