[Feature] Optional prefix for URL path

[andzejsp]

As my journey ended with the proxypass thing, ( #76 ), i went through the code could not understand hot to modify the python parts to be able to define the /path/ for the base url in the Flask app part.

Later i was going through the .js parts and the path is hardcoded for the static files :(.

Its not trivial, because people might be running this on subdomain -gpt.example.com, but some people who dont have a free subdomain left might want to run this on sub path example.com/gpt.

Something to this : https://stackoverflow.com/questions/18967441/add-a-prefix-to-all-flask-routes