Add form for registered users to add ``Events``.

https://github.com/madewithbytes/us_ignite/issues/86

The form lists all the fields available to add an event,
the format of the datetime is expressed in the settings
file.
Add functionality to add Events.

https://github.com/madewithbytes/us_ignite/issues/86

The events can be added by registered users using the form
available at ``/event/add``. The status of the event is
``DRAFT`` or ``PUBLISHED``.
Add event link to the navigation.
Show help text for the format of the datetime.

https://github.com/madewithbytes/us_ignite/issues/86