Add X-Frame-Options header to admin app

We should only let ourselves be able to host this app in a frame. No one
should be able to click jack us.
Add clamav to frontend role

The admin app runs on frontend-app machines and requires clamav, we now
need this to exist.

We should be able to remove it off the backend machines when backdrop
admin is retired.
Merge pull request #543 from alphagov/add-clamav-to-frontend

Add clamav to frontend role
We need to add `add_header` to proxy_vhost

I forgot that our vhost definitions in YAML didn't use the
nginx::resource::vhost resource and instead used our proxy_vhost. This
mean that deployment broke because we were not exposing the add_header
parameter. This commit fixes that by passing it through to the
underlying.
Merge pull request #544 from alphagov/fix-add-header

We need to add `add_header` to proxy_vhost