By explicitly importing the legacy namespace classes, we make it clear
that we are using soon-to-be-deprecated classes. We will then be able to
start removing the legacy classes, one module at a time.
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.
