release: unify formatDate, add timezone conversion, issue dry-runs

Created by: bobheadxi

Closes https://github.com/sourcegraph/sourcegraph/issues/14626

Now renders dates in tracking issues like so:

## [Oct 20, 2020, 6:00 PM (UTC)](https://time.is/1800_20_Oct_2020_in_UTC?3.21%20release%20date): Release

A similar link is provided in Slack announcements

Also adds a dryRun.trackingIssues option which prints the rendered tracking issue to stdout instead of an actual issue