Reverts the removal of this from
https://github.com/discourse/discourse-calendar/pull/231. This opens up
the relative url `/discourse-post-event/events.ics` for access to an
ical formatted response.
We also make available the `order` argument for this endpoint which
defaults to ascending order if there is no value or invalid value
passed. This maintains existing behaviour while allowing for users to
specify that they want most recent events first.