Added Backup code verification logic

Also added testing to cover as part of this in addition to adding the
core backup code handling required.

Also added the standardised translations for switching mfa mode and
adding testing for this switching.

app/Http/Controllers/Auth/MfaController.php

@@ -5,7 +5,7 @@ namespace BookStack\Http\Controllers\Auth;
 use BookStack\Actions\ActivityType;
 use BookStack\Auth\Access\Mfa\MfaValue;
 use BookStack\Http\Controllers\Controller;
-use BookStack\Http\Request;
+use Illuminate\Http\Request;
 
 class MfaController extends Controller
 {
@@ -47,7 +47,6 @@ class MfaController extends Controller
      */
     public function verify(Request $request)
     {
-        // TODO - Test this
         $desiredMethod = $request->get('method');
         $userMethods = $this->currentOrLastAttemptedUser()
             ->mfaValues()