Add an absolute url mechanic for the ``Hub``.

Helper to determine the URL of a given hub.
Add ``HubMembership`` model to register users with a ``Hub``.

https://github.com/madewithbytes/us_ignite/issues/98

This model will keep track of the users that have registered
in a given community.
Add membershp URL mechanics to the ``Hub``.

https://github.com/madewithbytes/us_ignite/issues/98

The method is helpful to determine the URL required to
give ``Hub`` membership to a ``User``.
Add mechanics for ``Users`` to associate with a community.

https://github.com/madewithbytes/us_ignite/issues/98

The process is initiated from the ``Hub`` page, since
makes more for the user to subscribe to a commiunity
after the user has seen its content.

The view to add the membership is only available
via ``POST`` initiated from the ``Hub`` detail page.

The hub detail page determines when a user is the
guardian or a member of this community.