R1:1d73ae3b50a6

Fix two minor timezone display issues

Summary:
Ref T13263. Two minor issues:

- The "reconcile" dialog shows the wrong sign because JS signs differ from normal signs (for example, PST or PDT or whatever we're in right now is shown as "UTC+7", but should be "UTC-7").
- The big dropdown of possible timezones lumps "UTC+X:30" timezones into "UTC+X".

Test Plan:
- Reconciled "America/Nome", saw negative UTC offsets for "America/Nome" and "America/Los_Angeles" (previously: improperly positive).