PLT-1909 Removed the ability to edit system posts (#3013)

* Removed the ability to edit system posts

* Added increased timeout to SpinnerButton unit tests

api/post_test.go

@@ -155,6 +155,17 @@ func TestUpdatePost(t *testing.T) {
 			t.Fatal("failed to updates")
 		}
 	}
+
+	post3 := &model.Post{ChannelId: channel1.Id, Message: "a" + model.NewId() + "a", Type: model.POST_JOIN_LEAVE}
+	rpost3, err := Client.CreatePost(post3)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	up3 := &model.Post{Id: rpost3.Data.(*model.Post).Id, ChannelId: channel1.Id, Message: "a" + model.NewId() + " update post 3"}
+	if _, err := Client.UpdatePost(up3); err == nil {
+		t.Fatal("shouldn't have been able to update system message")
+	}
 }
 
 func TestGetPosts(t *testing.T) {