FIX: limit number of users addable to group at once (#10510)

When someone wants to add > 1000 users at once they will hit a timeout.
Therefore, we should introduce limit and inform the user when limit is exceeded.

spec/requests/groups_controller_spec.rb

@@ -1245,6 +1245,33 @@ describe GroupsController do
             count: 3
           ))
         end
+
+        it 'display error when try to add to many users at once' do
+          begin
+            GroupsController.send(:remove_const, "ADD_MEMBERS_LIMIT")
+            GroupsController.const_set("ADD_MEMBERS_LIMIT", 4)
+            user1.update!(username: 'john')
+            user2.update!(username: 'alice')
+            user3 = Fabricate(:user, username: 'bob')
+            user4 = Fabricate(:user, username: 'anna')
+            user5 = Fabricate(:user, username: 'sarah')
+
+            expect do
+              put "/groups/#{group.id}/members.json",
+                params: { user_emails: [user1.email, user2.email, user3.email, user4.email, user5.email].join(",") }
+            end.to change { group.users.count }.by(0)
+
+            expect(response.status).to eq(422)
+
+            expect(response.parsed_body["errors"]).to include(I18n.t(
+              "groups.errors.adding_too_many_users",
+              limit: 4
+            ))
+          ensure
+            GroupsController.send(:remove_const, "ADD_MEMBERS_LIMIT")
+            GroupsController.const_set("ADD_MEMBERS_LIMIT", 1000)
+          end
+        end
       end
 
       it "returns 422 if member already exists" do