Add command to set timezone role prefix

[generic-github-user]

A prefix is added to each role name before the timezone name to distinguish timezone roles from all other roles. This was added to prevent an issue where a role like FIRST! was converted to a timezone role (like IRST - 5:43 PM) whenever updateTimes() or updateAllTimes().

It would be nice if users could customize this prefix using a command, or even add a suffix. More complicated identifiers could potentially be added using regular expression checking.

An alternate option would be to keep a running list of role IDs for each server/guild, but this would be memory-expensive and outweigh the costs of having a short role prefix or suffix on each role.