extensionSystem: Allow disabling session mode extensions

extensionSystem: Allow disabling session mode extensions

Trying to disable an extension that is enabled by the session mode
currently has no effect, which is clearly confusing. We could update
the various extension UIs to reflect that via sensitivity, but being
unable to configure extensions based on which session the user picked
at login isn't obvious either.

So instead, add a 'disabled-extensions' gsettings key to list extensions
that should not be enabled which takes precedence over 'enabled-extensions'
and can be used to disable session mode extensions.

https://bugzilla.gnome.org/show_bug.cgi?id=789852