Add support for reverse proxy about chatpad

[zer0ish]

I'm behind a reverse proxy and have no issues.

Pfsense for router with Cloudflare, server is unraid and using Nginx Proxy Manager.
I use ai.mydomain.net where mydomain.net is the main website.
Also had no issues using HAProxy on pfsense.

[T3rm1]

What you do is something different. Of course it works behind a reverse proxy and a separate sub domain. But it won't work if you require a non root path.

[ItzDerock]

@T3rm1 Are you able to see if my PR fixes your issue?
My PR adds an environment variable called BASEPATH that can be set, for example, to /chatpadai.
This will update the site <base> tag and lets react router know of the new basepath.

It will not serve the files for you under the new basepath though, you will have to configure a reverse proxy to do that for you. Let me know if this is a problem, and I can make it update the container's webserver to serve under the specified base path.

[T3rm1]

I checkout out your branch and tested it but it doesn't work. JS and CSS files are sill requested from root path which won't work.