Add forward proxy and proxyconfig

[mstormi]

If installed, nginx in openhabian can also provide a forward proxy. This can be used to access devices in a remote location via openHABian box serving as a proxy in that location. Users would need to manually configure this a the proxy in his browser. As an enhancement, we maybe could build a browser auto proxy config that considers the remote network (to be extracted from the network interface config?) as destinations it would enforce proxying for while anything else will be accessed directly. Need to place proxy autoconfig as a file /etc/openhab/html/proxyconf.pac which is served as http://\<openhabian>/static/proxyconf. Take care it needs to be served with a specific mime type application/x-ns-proxy-autoconfig.

• [x] nginx config enhancement see #1591
• [ ] proxy autoconfig file

[mstormi]

On 2nd thought the proxyconfig is a little over the top. Will hence close this for now, maybe some day when we're out of work left we can reactivate it