PLT-3077 Add group messaging (#5489)

* Implement server changes for group messaging

* Majority of client-side implementation

* Some server updates

* Added new React multiselect component

* Fix style issues

* Add custom renderer for options

* Fix model test

* Update ENTER functionality for multiselect control

* Remove buttons from multiselect UI control

* Updating group messaging UI (#5524)

* Move filter controls up a component level

* Scroll with arrow keys

* Updating mobile layout for multiselect (#5534)

* Fix race condition when backspacing quickly

* Hidden or new GMs show up for regular messages

* Add overriding of number remaining text

* Add UI filtering for team if config setting set

* Add icon to channel switcher and class prop to status icon

* Minor updates per feedback

* Improving group messaging UI (#5563)

* UX changes per feedback

* Update email for group messages

* UI fixes for group messaging (#5587)

* Fix missing localization string

* Add maximum users message when adding members to GM

* Fix input clearing on Android

* Updating group messaging UI (#5603)

* Updating UI for group messaging (#5604)

api/channel_test.go

@@ -219,6 +219,46 @@ func TestCreateDirectChannel(t *testing.T) {
 	}
 }
 
+func TestCreateGroupChannel(t *testing.T) {
+	th := Setup().InitBasic()
+	Client := th.BasicClient
+	user := th.BasicUser
+	user2 := th.BasicUser2
+	user3 := th.CreateUser(Client)
+
+	userIds := []string{user.Id, user2.Id, user3.Id}
+
+	var channel *model.Channel
+	if result, err := Client.CreateGroupChannel(userIds); err != nil {
+		t.Fatal(err)
+	} else {
+		channel = result.Data.(*model.Channel)
+	}
+
+	if channel.Type != model.CHANNEL_GROUP {
+		t.Fatal("channel type was not group")
+	}
+
+	// Don't fail on group channels already existing and return the original channel again
+	if result, err := Client.CreateGroupChannel(userIds); err != nil {
+		t.Fatal(err)
+	} else if result.Data.(*model.Channel).Id != channel.Id {
+		t.Fatal("didn't return original group channel when saving a duplicate")
+	}
+
+	if _, err := Client.CreateGroupChannel([]string{user.Id}); err == nil {
+		t.Fatal("should have failed with not enough users")
+	}
+
+	if _, err := Client.CreateGroupChannel([]string{}); err == nil {
+		t.Fatal("should have failed with not enough users")
+	}
+
+	if _, err := Client.CreateGroupChannel([]string{user.Id, user2.Id, user3.Id, "junk"}); err == nil {
+		t.Fatal("should have failed with non-existent user")
+	}
+}
+
 func TestUpdateChannel(t *testing.T) {
 	th := Setup().InitBasic().InitSystemAdmin()
 	Client := th.SystemAdminClient