Add timezone preference to `/account` page

[Marak]

Users should have the option to set their own timezone. The value could be auto-detected on login and then also be manually set on the /account page.

This will be useful for several of the pages on the websites as well as some of the API calls. The result will be that users gain the ability to see logs / crons / mtimes / etc in their own timezone.

Currently I believe everything is hard-coded to EDT timezone.