Bug 40925: Implemented the Security Level component

This component adds a new Security Level toolbar button which visually
indicates the current global security level via icon (as defined by the
extensions.torbutton.security_slider pref), a drop-down hanger with a
short description of the current security level, and a new section in
the about:preferences#privacy page where users can change their current
security level. In addition, the hanger and the preferences page will
show a visual warning when the user has modified prefs associated with
the security level and provide a one-click 'Restore Defaults' button to
get the user back on recommended settings.

Bug 40125: Expose Security Level pref in GeckoView