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.
If I can hearing that right, then an admin would have to configure:

```
:features:
  :products-discovery-create: false
  :products-discovery-scan: false
  :products-new-form: false
```

And then how would that propogate to be used in HTML for say the repo-discovery button as per the example?

Or are you thinking of a pre-existing mapping somewhere in the JS code? e.g.

```
stateFeatureMap = {
    'custom_products': ['products.discovery.create', 'products.discovery.scan', 'products.new.form']
}
```