Fix AppRegistryNotReady error for panels that load models.

[vdboor]

The import to the debug_toolbar.middleware also loads all panels.
As a panel might be using Django models, such import happens too early.

This pull request moves the import statement so these errors won't occur.
The backtrace looks like:

File "django/__init__.py", line 18, in setup
  apps.populate(settings.INSTALLED_APPS)
File "django/apps/registry.py", line 85, in populate
  app_config = AppConfig.create(entry)
File "django/apps/config.py", line 116, in create
  mod = import_module(mod_path)
File "importlib/__init__.py", line 37, in import_module
  __import__(name)
File "debug_toolbar/apps.py", line 12, in <module>
  from debug_toolbar.middleware import DebugToolbarMiddleware
File "debug_toolbar/middleware.py", line 17, in <module>
  from debug_toolbar.toolbar import DebugToolbar
File "debug_toolbar/toolbar.py", line 151, in <module>
  urlpatterns = DebugToolbar.get_urls()
File "debug_toolbar/toolbar.py", line 145, in get_urls
  for panel_class in cls.get_panel_classes():
File "debug_toolbar/toolbar.py", line 118, in get_panel_classes
  mod = import_module(panel_module)
File "importlib/__init__.py", line 37, in import_module
  __import__(name)
File "fluent_contents/panels.py", line 7, in <module>
  from django.contrib.contenttypes.models import ContentType

[codecov-io]

Current coverage is 77.00% (diff: 100%)

Merging #889 into master will not change coverage

@@             master       #889   diff @@
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  Files            30         30          
  Lines          1635       1635          
  Methods           0          0          
  Messages          0          0          
  Branches        244        244          
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  Hits           1259       1259          
  Misses          302        302          
  Partials         74         74          

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