rails 5: only include well-known zones in the US time zone list
rails 5: only include well-known zones in the US time zone list

refs CNVS-38321

test plan:
 * go to account settings
 * open the time zone dropdown
 * there shouldn't be a bunch of zones at the top without translations

Change-Id: Ib3d641f641c0773e510eb36f3e85fba3c257a036
Reviewed-on: https://gerrit.instructure.com/122163
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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