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

- 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.