Add reverse-proxy subpath support via BASE_PATH env var

[drjaska]

Added support for running subsyncarr under a reverse-proxy subpath by adding a new BASE_PATH env var for configuration.

• Added a configuration mention in README.md
• Handled /api/* routes, static assets, and WebSocket
• Injected window.__BASE_PATH__ into index.html as a prerequisite for stuff to work.

I didn't catch any regressions for those users who do not set the BASE_PATH env var but I didn't test that thoroughly.

The following Caddyfile for https://example.com/subsyncarr/ should now just work without the need to strip the subpath while proxying.

example.com {
    handle /subsyncarr/* {
        reverse_proxy localhost:3000
    }
}

In the case that I missed something, feel free to let me know!