Store and handle a user TimeZone setting

[DavidRieman]

When the server sends clock time displays (such as for account creation history display, or a "give me the current time" command or message board post dates and the like), it may be ok to default to sending the date/time via the server's time zone. However, the user should be allowed to optionally specify their time zone so that the server can convert the date/time to the user's time zone.

For this ticket, we should:

• The user's TimeZone should probably be stored somewhere within the User object (not just the Player). (Ensure the format stored will be brief but helpful for standard DateTime conversions).
• Add a command to set the TimeZone.
• Look for existing DateTime presentations to adjust to the presentation taking the user's TimeZone into consideration with standard DateTime conversion means. (No funky math; don't roll your own!) (If this is too much, file a separate ticket for this part instead.)