Make ``Domain`` and ``Feature`` slugs unique.

Since the ``slugs`` are used to filter data it is
necessary that they are unique.
Add ``get_domain`` test fixture factory.

Helpful to test the ``Domain``.
Increase the number of dummy ``Applications`` created.
Add section to filter ``Applications`` by ``Domain``.

https://github.com/madewithbytes/us_ignite/issues/135

A section has been added to filter the ``Applications`` by
``Domain``.

It uses the ``slug`` of the domain to filter. Reuses the
``app_list`` view.