PLT-6554 Add config setting to control enabling API version 3 (#6835)

* Add config setting to control enabling API version 3

* Update help text for APIv3 config setting (#6843)

* Update configuration_settings.jsx

* Update en.json

api/admin_test.go

@@ -4,6 +4,7 @@
 package api
 
 import (
+	"net/http"
 	"strings"
 	"testing"
 
@@ -652,3 +653,24 @@ func TestGetRecentlyActiveUsers(t *testing.T) {
 		t.Fatal("should have been at least 2")
 	}
 }
+
+func TestDisableAPIv3(t *testing.T) {
+	th := Setup().InitBasic()
+	Client := th.BasicClient
+
+	enableAPIv3 := *utils.Cfg.ServiceSettings.EnableAPIv3
+	defer func() {
+		*utils.Cfg.ServiceSettings.EnableAPIv3 = enableAPIv3
+	}()
+	*utils.Cfg.ServiceSettings.EnableAPIv3 = false
+
+	_, err := Client.GetUser(th.BasicUser.Id, "")
+
+	if err.StatusCode != http.StatusNotImplemented {
+		t.Fatal("wrong error code")
+	}
+
+	if err.Id != "api.context.v3_disabled.app_error" {
+		t.Fatal("wrong error message")
+	}
+}