Add navigation link to view Ignite communities.
Add ``Hub`` method to record activity.

https://github.com/madewithbytes/us_ignite/issues/93

The ``HubActivity`` records an stream of content generated
by the community.
Register as a hub activity when an app is part of a Hub.

https://github.com/madewithbytes/us_ignite/issues/93

Whenever an application has registered as part of a ``Hub``
it will be recorded as part of the hub activity.