Added owner property to match generic objects signature.
Transform the membership list into a user list.

https://github.com/madewithbytes/us_ignite/issues/220

This will help to reuse the generic sidebar supporters list.
Add button to support/subscribe a Hub.

https://github.com/madewithbytes/us_ignite/issues/220

The button is only shown when the user is not the owner
or not a member of this Hub.