Offer feature switch to turn off link to our webform

This adds a `SHOW_WEBFORM` setting. If set to True, the app will display the behavior it has been, which is to link to our basic webform whenever the agency/office does not provide one. If set to False, the app will demonstrate new behavior, which is to hide the webform section entirely if the agency/office does not provide one.

This allows us to pass the app around publicly a little more easily, since the webform still has some more work to be done before it's fully operational.

Fixes #250.
I wonder if we should name this: SHOW_REQUEST_FORM.