Allow iCal feeds to report due dates in a configured timezone

[coddingtonbear]

Right now, the iCal feed simply truncates the raw due or waiting field to its date. This means that due dates will accidentally be correct as long as you are in a timezone west of UTC, but if you are in a timezone east of it, your dates will be off by a day.

There are two obvious paths for fixing this:

• Update UI for iCal feed such that you can specify a timezone for your generated feed. We'd then look up that timezone and transform the due/wait date into that timezone before truncating it to its date. This is probably the ideal solution. We may also want to surface this timezone selection higher up in the UI because it could be useful for other things, too. For existing accounts, we could set this option to UTC.
• Add (optional?) request parameters for specifying a timezone -- possibly as path components, or possibly as URL parameters (e.g.?tz=Europe/Stockholm) that will be read from for performing the same translation described above. This is the easiest solution, but would require that folks take care to do this and, really, the documentation burden is probably more effort than just doing this the right way.