installer: add wizard page to install for all users on windows

this adds a custom installer page with a checkbox to allow users to
choose to install the app for all users, when its enabled installer
will create a group called 'OpenVPN Administrator'

local users except the admin, guest and default account are added to
the group, and upon uninstall the group is removed

this allows non-admin users that are added to the group to use the app
and successfully start a vpn connection