title: Django admin CheckboxSelectMultiple widget not working for ManyToManyField
body: <p>I'm using the below model where the default django users table has a many-to-many relationship with Hotel's.  </p>
  
  <p>Assigning multiple users to a Hotel in the django admin panel is difficult, and I want to replace the default 'ctrl-click' method to a checkbox where I can select multiple users without having to hold down ctrl.  </p>
  
  <p>Model:</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import User
  
  class Hotel(models.Model):
      # associate the user
      user = models.ManyToManyField(User)
      # additional fields
      hotel_name = models.CharField(max_length=100)
      hotel_id = models.CharField(max_length=100)
  
      def __str__(self):
          return self.hotel_id
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>from django.contrib import admin
  from .models import *
  from django.forms import CheckboxSelectMultiple
  
  
  # Checkbox for many-to-many fields
  class HotelAdmin(admin.ModelAdmin):
      formfield_overrides = {
          models.ManyToManyField: {'widget': CheckboxSelectMultiple},
      }
  
  admin.site.register(Hotel)
  </code></pre>
  
  <p>result:
  The formfield_override does not work, and selecting users to a hotel is not a checkbox
  <a href="https://i.stack.imgur.com/WrKq2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WrKq2.png" alt="enter image description here"></a></p>