PLT-6714: add /settings command (#6716)

* add /settings command

* update receiver name

api/command_settings_test.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api
+
+import (
+	"testing"
+)
+
+func TestSettingsCommand(t *testing.T) {
+	th := Setup().InitBasic()
+	th.BasicClient.Must(th.BasicClient.Command(th.BasicChannel.Id, "/settings"))
+}