In Calendar ICS imports, parse "Etc/GMT" as "UTC"
Summary: Ref T11801. This is a legacy timezone which is effectively equivalent to "UTC" (they differ by no more than a second, owing to variations in the speed of the earth's rotation).
Test Plan:
- Ran unit tests.
- Faked an entry in the map to make a test fail.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11801
Differential Revision: https://secure.phabricator.com/D16812