Fix some bugs with too early imports

biscuit/apps/mensa/migrations/0001_initial.py

 # Generated by Django 2.2.1 on 2019-05-29 15:05
 
 from django.db import migrations, models
-import helper
+import utils.helper as helper
 
 
 class Migration(migrations.Migration):

biscuit/apps/mensa/models.py

 from django.db import models
 
 # Create your models here.
-from helper import path_and_rename
+from utils.helper import path_and_rename
 
 
 class Menu(models.Model):