Add setting for custom nginx config

malrequested to merge
half-duplex/ansible:ngx-custom-config into master

In my vhosts, I use an included file for an ACME location block. This PR adds the ability to append arbitrary nginx config to the ansible-installed vhost, which allows me to do what I need to do without needing to maintain my own copy of the entire vhost.

It adds a config var, funkwhale_nginx_additional_config, with an empty default.

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