Require acceptance of the rules before adding an emote for the first time

[ioistired]

• add a rules string to the bot's config
• if configured, it would be displayed by ec/add when the user has not yet agreed to the rules. The user must accept in order to add an emote.
• If they do not agree, cancel the ec/add command.
• Otherwise, add their user ID to an "agreed_users" single-column table
• users can view the rules at any time using the ec/rules command.