Maintenance banner functionality, including a new waffle switch and setting

This adds the following new waffle switch (and namespace):

open_edx_util.display_maintenance_warning

and a new setting:

settings.MAINTENANCE_BANNER_TEXT

The provided view decorator allows developers to select which views
receive the static banner, and the banner can be globally toggled via
the waffle switch.  The text of the banner is determined by the setting,
which is currently not translated.

openedx/core/djangoapps/util/maintenance_banner.py

@@ -0,0 +1,43 @@
+"""
+View decorator to add a maintenance banner configured in settings.
+"""
+from __future__ import absolute_import
+
+from functools import wraps
+
+from django.conf import settings
+
+from openedx.core.djangoapps.util.user_messages import PageLevelMessages
+from openedx.core.djangoapps.util.waffle import DISPLAY_MAINTENANCE_WARNING, waffle
+
+
+def add_maintenance_banner(func):
+    """
+    View decorator to select where exactly the banner will appear
+
+    Add to function-based views like this:
+
+        from openedx.core.djangoapps.util.maintenance_banner import add_maintenance_banner
+
+        @add_maintenance_banner
+        def my_view(request):
+            ...
+
+    Add to class-based views using method_decorator:
+
+        from openedx.core.djangoapps.util.maintenance_banner import add_maintenance_banner
+        from django.utils.decorators import method_decorator
+
+        @method_decorator(add_maintenance_banner, name='dispatch')
+        class MyView(View):
+            ...
+    """
+    @wraps(func)
+    def _decorated(request, *args, **kwargs):  # pylint: disable=missing-docstring
+        if waffle().is_enabled(DISPLAY_MAINTENANCE_WARNING):
+            if hasattr(settings, 'EDXAPP_MAINTENANCE_BANNER_TEXT') and settings.EDXAPP_MAINTENANCE_BANNER_TEXT:
+                # The waffle switch is enabled and the banner text is defined
+                # and non-empty.  We can now register the message:
+                PageLevelMessages.register_warning_message(request, settings.EDXAPP_MAINTENANCE_BANNER_TEXT)
+        return func(request, *args, **kwargs)
+    return _decorated

openedx/core/djangoapps/util/waffle.py

@@ -0,0 +1,18 @@
+"""
+Waffle flags and switches
+"""
+from __future__ import absolute_import
+
+from openedx.core.djangoapps.waffle_utils import WaffleSwitchNamespace
+
+WAFFLE_NAMESPACE = u'open_edx_util'
+
+# Switches
+DISPLAY_MAINTENANCE_WARNING = u'display_maintenance_warning'
+
+
+def waffle():
+    """
+    Returns the namespaced, cached, audited Waffle class for open_edx_util.
+    """
+    return WaffleSwitchNamespace(name=WAFFLE_NAMESPACE, log_prefix=u'OpenEdX Util: ')