Set up NGINX proxy cache

[kysrpex]

Configure proxy_cache_path and define a macro upstream_galaxy_cache() that allows NGINX to cache content by itself for a given location, so that requests do not have to be sent to the (slow Python) upstream. Use it to cache the favicon and /static.

Using this approach, static files are still served by Galaxy, which knows best where to find them (especially regarding edge cases, such as per-subdomain static files); but NGINX caches them so that gunicorns have to work very little (once every 30m).

Closes https://github.com/usegalaxy-eu/issues/issues/919, https://github.com/usegalaxy-eu/issues/issues/927. Supersedes #2001.

⚠️ The Galaxy EU favicon may be replaced with the Galaxy Project favicon unless #1996 is merged too.

[kysrpex]

If you like this idea we could integrate nginx_cache_dir into usegalaxy-eu.nginx.

    - role: nginx_cache_dir
      vars:
        nginx_cache_dir_owner: root
        nginx_cache_dir_group: root
        nginx_cache_dir_mode: "0777"
    - usegalaxy-eu.nginx
    - nginx_cache_dir