Being able to specify a context path

[rkrombho]

Running Descartes behind a reverse proxy where you want to define the context path (like e.g. /descartes) won't really work as all links are hardcoded in an absolute way.

It would be great if the context path could be configured inside the .env file.

I did the necessary changes in my fork (https://github.com/rkrombho/descartes/commit/f3172a0e120351e8d4da4649a70c6874b181d0ed) but don't really feel confident with raising this as a pull request as I don't know much about Ruby and the Descarted codebase at all. Maybe a good starting point.

[smith3v]

+1. Would be a good enhancement.