Modify Add-on support

Bug 40030: Install HTTPS Everywhere and NoScript addons on startup

HTTPS Everywhere is installed as a builtin extension and NoScript as
a regular AMO addon. To avoid unnecessary I/O we only install NoScript
the first time, and rely on the browser addon updating mechanism for
keeping up with new versions. This is the same behaviour that was
implemented in the Fennec-based Tor Browser, where it was installed
as a "distribution addon", which also only occurred once.

Bug 40062: HTTPS Everywhere is not shown as installed

Also 40070: Consider storing the list of recommended addons

This implements our own AddonsProvider, which loads the list of
available addons from assets instead of fetching it from an
endpoint. In this list, we replace https-everywhere by
our https-everywhere-eff, so that the EFF one is shown as installed
in the addons list and the AMO one is not displayed.

Also, we hide the uninstall button for builtin addons.

Bug 40058: Hide option for disallowing addon in private mode