Experimental HTTP2 support

A simple script that backports nginx and openssl from Debian Stretch and activate nginx’s HTTP2 support : https://github.com/Bouska/http2_ynh . It’s suppose to be a workaround until Yunohost is rebased on Debian Stretch. It has only being tested by me until now.

To test it, install the app and test if your server support HTTP2 with this tool : https://tools.keycdn.com/http2-test