Implement optional url prefix the Django way

* Before, the optional url prefix feature required calling our
  versioning version of reverse(). This worked _ok_ until we added more
  and more urls from 3rd party apps. Those 3rd party apps do not call
  our reverse(), writefully so.
* This implementation looks at the incoming request path. If it includes
  the special optional prefix url, then we register ALL the urls WITH
  the optional url prefix.
  If the incoming request path does NOT contain the options url prefix
  then we register ALL the urls WITHOUT the optional url prefix.
* Before this, we were registering BOTH sets of urls and then reverse()
  + the request as context to decide which url.

awx/settings/defaults.py

@@ -1002,6 +1002,7 @@ MIDDLEWARE = [
     'django.contrib.auth.middleware.AuthenticationMiddleware',
     'awx.main.middleware.DisableLocalAuthMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
+    'awx.main.middleware.OptionalURLPrefixPath',
     'awx.sso.middleware.SocialAuthMiddleware',
     'crum.CurrentRequestUserMiddleware',
     'awx.main.middleware.URLModificationMiddleware',