Add form to manage hub-app memberships.

https://github.com/madewithbytes/us_ignite/issues/23

The form will list all the available ``Hubs``.
Add model ``HubAppMembership`` to track app-hub relationship.

https://github.com/madewithbytes/us_ignite/issues/23

The model keeps ``is_featured`` and ``created`` date as
metadata of the relationship.
Add view mechanics to associate hubs-apps.

https://github.com/madewithbytes/us_ignite/issues/23

The view manages the app-hubs relationship. Using private
helper functions for this. Since the relationship is maintained
in a through table.
List the hub associations in an app detail page.

https://github.com/madewithbytes/us_ignite/issues/23

The associations are shown in the detail page.