Probably fix some display issues with all-day events?

Summary:
Ref T11801. These are pretty fiddly because users expect to see the end time for timed events ("10 AM - 11 AM" is ONE hour long) but not for all-day events ("Nov 2 - Nov 3" is TWO days long!)

We also want to store the thing the user actually entered so we don't lose data if they un-all-day the event later.

This may take a little more fiddling since it feels a little shaky, but I couldn't break this version immediately.

Test Plan: Imported a French holiday, got proper display in the UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16815