Currently extensions can only be locked down completely by
restricting the `enabled-extensions` key via dconf.
This is too restrictive for environments that want to allow users
to customize their system with extensions, while still limiting
the set of possible extensions.
To fill that gap, add a new `allow-extension-installation` setting,
which restricts extensions to system extensions when disabled.
As the setting is mainly intended for locking down by system
administrators, there is no attempt to load/unload extensions
on settings changes.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3180>
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