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