Move nginx config files to subdirectory

[alxndrsn]

• tidy up the repo a little
• prepare for potential changes to the config in e.g. Procfile: run nginx via docker #1376

What has been done to verify that this works as intended?

Nothing - I can't currently start nginx locally from the Procfile.

Why is this the best possible solution? Were any other approaches considered?

Discussed at #1376 (comment)

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No change - dev only.

Does this change require updates to user documentation? If so, please file an issue here and include the link below.

No - the change is dev only.

Before submitting this PR, please make sure you have:

• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

Given that the command in the Procfile is still -p "$PWD", do any file paths need to change in main.nginx.conf? For example, for something like:

include ./common-headers.nginx.conf;

Isn't ./ still the top-level directory of the repository? I'm wondering whether that line needs to change to:

include ./nginx-conf/common-headers.nginx.conf;

Or are include paths relative to the config file rather than the -p path?

Would it help anything to change the -p path to "$PWD/nginx-conf"?

[alxndrsn]

Is there any chance you could give this branch a quick try? Given the motivation for this PR was #1376, which was motivated by not being able to run nginx -p in the first place...