When the JS "Intl" API is available, use it to guess the timezone

Summary:
Ref T3025. Chrome gives us an easily-accessible, much better guess at which timezone the user is in.

Firefox also exposes "Intl" but this doesn't seem to be a reliable method to read the timezone.

Test Plan:
In Chrome, swapped my system date/time between zones, clicked the "reconcile" popup, got the dropdown prefilled accurately.

In Safari (no Intl API) got the normal flow with no default selected.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3025

Differential Revision: https://secure.phabricator.com/D15962