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).
• Viewed the big timzone list, saw ":30" and ":45" timezones grouped/labeled more accurately.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13263

Differential Revision: https://secure.phabricator.com/D20314