Add web.config for IIS deployment of the client

[emmguyot]

This merge request adds the necessary web.config file to deploy the client as static files in IIS (windows).

It also modifies the way the base URL is set in index.html.

Now 3 parameters are set for the client in .env : REACT_APP_SERVER_BASE_URL : URL to access planka server API PUBLIC_URL : URL to access planka client (i.e.: where the client is deployed and accessed) BASE_URL : Base directory of the public URL with an ending /