Add command to join contests

[avatarkava]

Need to

• add table to store contest entrants as well as contests
• Require specific command (eg !join)
• Give periodic reminders (1 min remaining, etc) is a nice feature to add

This will eliminate a couple issues:

• users who don't want to participate won't have to be silent while the contest is running
• people who recently joined the room won't be automatically entered