Add mechanics for ``Event`` owners to edit its details.

https://github.com/madewithbytes/us_ignite/issues/107

The ``Events`` can be edited using the ``EventForm``
provided it is available from the ``Event`` detail
page.
Show ``edit`` link in the ``Event`` detail page.

https://github.com/madewithbytes/us_ignite/issues/107

The link is only available to the owner of the
``Event``.