Made daphne installable as a Django app.

Added system check to ensure daphne is installed before
django.contrib.staticfiles.

daphne/apps.py

@@ -0,0 +1,20 @@
+# Import the server here to ensure the reactor is installed very early on in case other
+# packages import twisted.internet.reactor (e.g. raven does this).
+import daphne.server  # noqa: F401
+
+
+try:
+    # Allow that Django is not installed.
+    from django.apps import AppConfig
+    from django.core import checks
+    from .checks import check_daphne_installed
+except ImportError:
+    pass
+else:
+
+    class DaphneConfig(AppConfig):
+        name = "daphne"
+        verbose_name = "Daphne"
+
+        def ready(self):
+            checks.register(check_daphne_installed, checks.Tags.staticfiles)

daphne/checks.py

@@ -0,0 +1,20 @@
+# Django system check to ensure daphne app is listed in INSTALLED_APPS before django.contrib.staticfiles.
+from django.core.checks import Error, register
+
+
+@register()
+def check_daphne_installed(app_configs, **kwargs):
+    from daphne.apps import DaphneConfig
+    from django.apps import apps
+    from django.contrib.staticfiles.apps import StaticFilesConfig
+
+    for app in apps.get_app_configs():
+        if isinstance(app, DaphneConfig):
+            return []
+        if isinstance(app, StaticFilesConfig):
+            return [
+                Error(
+                    "Daphne must be listed before django.contrib.staticfiles in INSTALLED_APPS.",
+                    id="daphne.E001",
+                )
+            ]