title: Dependency on app with no migrations: %s" % key[0]) in the Django
body: <p>I need create custom user. I'm using a user model with AbstractUser:</p>
  
  <pre><code>from django.contrib.auth.models import AbstractUser
  from django.db import models
  from django.utils.html import escape, mark_safe
  
  
  class User(AbstractUser):
      is_student = models.BooleanField(default=False)
      is_teacher = models.BooleanField(default=False)
  </code></pre>
  
  <p>settings.py:</p>
  
  <pre><code>INSTALLED_APPS = [
      'django.contrib.auth',
      'django.contrib.contenttypes',
      'django.contrib.sessions',
      'django.contrib.messages',
      'django.contrib.staticfiles',
      'django.contrib.humanize',
  
      'crispy_forms',
  
      'backend.classroom',
  ]
  </code></pre>
  
  <p>I believe that problem is in the following line:</p>
  
  <pre><code>AUTH_USER_MODEL = 'classroom.User'
  </code></pre>
  
  <p>error:</p>
  
  <pre><code>File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 82, in handle
      executor = MigrationExecutor(connection, self.migration_progress_callback)
    File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/db/migrations/executor.py", line 18, in __init__
      self.loader = MigrationLoader(self.connection)
    File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/db/migrations/loader.py", line 49, in __init__
      self.build_graph()
    File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/db/migrations/loader.py", line 226, in build_graph
      self.add_external_dependencies(key, migration)
    File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/db/migrations/loader.py", line 191, in add_external_dependencies
      parent = self.check_key(parent, key[0])
    File "/home/davi/.local/share/virtualenvs/django-vue-template-Wl6a6m2J/lib/python3.6/site-packages/django/db/migrations/loader.py", line 173, in check_key
      raise ValueError("Dependency on app with no migrations: %s" % key[0])
  ValueError: Dependency on app with no migrations: classroom
  </code></pre>
  
  <p>obs: The app classrom app is in the backend folder. I tried the following code too:</p>
  
  <pre><code>AUTH_USER_MODEL = 'backend.classroom.User'
  </code></pre>