Fixes #8560: Allow fencing HTML and routes off via a feature flag.

The feature flag can be set in the Foreman settings.yaml file or
in a plugin settings within config/settings.plugins.d directory. The
settings format is:

:features:
  :custom_products: false

See the feature flag documentation for how to use it within HTML
and on a route.
I think this is a fair solution to the "competing concerns" issue brought up in https://github.com/Katello/bastion/pull/28.