Add system check to confirm that compiled CSS files are present.

Fixes #1720. Thanks to Shu Ishida for initial development on this feature.

wagtail/wagtailadmin/apps.py

@@ -1,5 +1,7 @@
 from django.apps import AppConfig
 
+from . import checks  # NOQA
+
 
 class WagtailAdminAppConfig(AppConfig):
     name = 'wagtail.wagtailadmin'

wagtail/wagtailadmin/checks.py

@@ -0,0 +1,30 @@
+import os
+
+from django.core.checks import Error, register
+
+
+@register()
+def css_install_check(app_configs, **kwargs):
+    errors = []
+
+    css_path = os.path.join(
+        os.path.dirname(__file__), 'static', 'wagtailadmin', 'css', 'normalize.css'
+    )
+
+    if not os.path.isfile(css_path):
+        error_hint = """
+            Most likely you are running a development (non-packaged) copy of
+            Wagtail and have not built the static assets -
+            see http://docs.wagtail.io/en/latest/contributing/developing.html
+
+            File not found: %s
+        """ % css_path
+
+        errors.append(
+            Error(
+                "CSS for the Wagtail admin is missing",
+                hint=error_hint,
+                id='wagtailadmin.E001',
+            )
+        )
+    return errors