Timezone setting

[finnbar]

Timezones are a pain, because not only do they vary per country, but they also change. We currently default to UTC, which is correct for half of the time in the UK. Two possible solutions to this:

1. A !timezone command which lets an admin set the bot's timezone. The challenge here is making sure that all plugins that use time respect this. You'll likely need some internal timezone helper file that provides plugins with this information (in an IORef?), and this command can mutate that information.
2. See if we can specify the timezone based on location. This will likely still need an internal timezone helper file like in (1).

[finnbar]

Low priority, since the bot is currently used in a UK server only, and many people use relative dates rather than absolute ones for reminders.