NGINX fixes

What does this MR do?

I did a fresh source install yesterday and noticed to problems:

1. The gitlab-ssl config doesn't pass nginx -t (with NGINX 1.14.2 on Debian buster)

nginx: [emerg] a duplicate default server for 0.0.0.0:80 in /etc/nginx/sites-enabled/gitlab:75
nginx: configuration file /etc/nginx/nginx.conf test failed

2. NGINX gets a permission denied on gitlab-workhorse.socket and GitLab's assets.

2021/03/18 18:54:15 [crit] 2095#2095: *1 connect() to unix:/var/lib/git/gitlab/tmp/sockets/gitlab-workhorse.socket failed (13: Permission denied) while connecting to upstream, client: <snip client IP>, server: <snip gitlab host>, request: "GET / HTTP/1.1", upstream: "http://unix:/var/lib/git/gitlab/tmp/sockets/gitlab-workhorse.socket:/", host: "<snip gitlab host>"
2021/03/18 18:54:15 [error] 2095#2095: *1 open() "/var/lib/git/gitlab/public/502.html" failed (13: Permission denied), client: <snip client IP>, server: <snip gitlab host>, request: "GET / HTTP/1.1", upstream: "http://unix:/var/lib/git/gitlab/tmp/sockets/gitlab-workhorse.socket/", host: "<snip gitlab host>"

This MR attempts to fix both issues.

Does this MR meet the acceptance criteria?

Conformity

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