Improve the UI for selecting a guardian of a ``Hub``.

Using a ``raw_id_fielss`` eases the selection of a user
since it can be searched and more details are available
at glance.
Add ``Hub`` methods to determine publication and guardian.
Add view to show the detail of a community.

https://github.com/madewithbytes/us_ignite/issues/50

The detail page of a community is available once
it is published or if the user is the guardian of
the community.