Add active ``Hub`` manager to the model.

This will help to filter published ``Hubs`` as well as
being consistent with the rest of the codebase.
Clean up model data after tests have been executed.
Add visibility method to the ``Hub`` model.

Helpful to determine if a user can view a given Hub.
Use the ``active`` manager to list available ``Hubs``.

The view to list the hubs available has been switched to
use the manager.
Add search form to the ``Event`` search tag page.
Use an include template to display ``Hub`` listings.
Add mechanics to search Hubs by tag name.

https://github.com/madewithbytes/us_ignite/issues/143

Hubs are filtered by tag name using the ``tag_search``
function.