Extension overview - Os / Arch should be used in a generic way

[jcfr]

Within the slicerpackages plugin there is already code allowing to expose the list of Os/Arch to both Js and PHP.

Ideally, this code should be re-used in the slicerappstore module.

See https://github.com/midasplatform/slicerpackages/blob/master/controllers/ViewController.php#L181

May a component could be created ?

Note also that currently the list of Os/arch is hardcoded. In the near future this list will either be contain in its own table or will be generated from the existing data.

[jcfr]

Note that this issue is associated to milestone 2