title: Add more field in SignupForm using django-allauth
body: <p><strong>Models:</strong></p>
  
  <pre><code>class Profile(models.Model):
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      bio = models.TextField(max_length=500, blank=True)
      nationality = models.CharField(max_length=20)
  
      def __str__(self):
          return self.user.first_name
  
      @receiver(post_save, sender=User)
      def create_user_profile(self, sender, instance, created, **kwargs):
          if created:
              Profile.objects.create(user=instance)
  
      @receiver(post_save, sender=User)
      def save_user_profile(self, sender, instance, **kwargs):
          instance.profile.save()
  </code></pre>
  
  <p><strong>Forms:</strong></p>
  
  <pre><code>from allauth.account.forms import SignupForm
  
  class CustomSignupForm(SignupForm):
      first_name = forms.CharField(max_length=100)
      last_name = forms.CharField(max_length=100)
  
      class Meta:
          model = Profile
          fields = ('first_name', 'last_name', 'nationality', 'bio')
  
      def signup(self, request, user):
          # Save your user
          user.first_name = self.cleaned_data['first_name']
          user.last_name = self.cleaned_data['last_name']
          user.save()
  
          user.profile.nationality = self.cleaned_data['nationality']
          user.profile.gender = self.cleaned_data['bio']
          user.profile.save()
  </code></pre>
  
  <p><strong>Views:</strong></p>
  
  <pre><code>ACCOUNT_FORMS = {'signup': 'myproject.forms.CustomSignupForm',}
  </code></pre>
  
  <p>This process isn't work. <strong>Error is: Model class all_auth.models.Profile doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.</strong></p>
  
  <p>How can I solve it? Or, How can i add more field with SignupForm using django-allauth?</p>