#514 Introduce Chrome web store protection

The amount of information sent is minimized without disabling
the ability to install extensions (which is under user control)
It became possible to activate the autoupdate of extensions by sending the minimum of values.
Installation with the chrome web store is allowed, but the website
by default cannot retrieve the list of installed extensions and their status (under flag)
All http calls related to requesting and downloading updates are cookie-free,
but the ip and the list of extensions are traceable, that information cannot be omitted.
Auto-update is disabled by default: it is possible to activate it with chrome://extensions/