When resolving a relative date, only ignore the viewer timezone if the date has a timezone

Summary: Ref T11801. In some cases, we generate a relative date without having a timezone. In these cases, use the viewer timezone.

Test Plan:

• Added a failing test case and made it pass.
• Imported the event from T11801#202293, which failed before this change and worked after it.
• Also added a reduced version of that event as a .ics test case since I caught a different bug. See next change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16974