add "description" blurb to INFO response and welcome message

Value

• as an admin of a channel, i would like to be able to offer more context about my channel to users before (or just after) they subscribe in an automated way, so that i don't have to repeat myself a lot
• as a potential subscriber to a channel, i would like to be able to learn more about what its contents are before subscribing (so i can offer more informed consent to join and not get spammed about things i don't care about)

Behavior

seeing a description

GIVEN a random person and a channel #foo

• WHEN the person subscribes to #foo
• THEN the channel's description will be included in the welcome message (if it exists)

GIVEN an unsubscribed (or subscribed) user alice on a channel that has a defined desription field

• WHEN alice issues INFO
• THEN the chanel responds with text that includes channel description: {text of the description} at the top of the message between the channel header and the stats list

GIVEN a user alice on a channel without a description

• WHEN alice issues INFO
• THEN alice receives a response that has no text between the header and the stats list

editing a description

channel GIVEN an admin alice on a channel #foo

• WHEN alice issues DESCRIPTION a channel about bar
• THEN the system will store "a channel about bar" as the channel description
• NOW WHEN alice issues INFO
• THEN she receives a message that includes description: a channel about bar

discovering the DESCRIPTION command

GIVEN an admin alice

• WHEN alice issues HELP
• THEN alice sees DESCRIPTION \n -> updates the description of this channel shown by INFO (or something like that) listed as one of the command explanations

Implementation notes

• add a description field to the channels table
• add a new DESCRIPTION command (in spanish and english!)
• add handlers for this command in commands/execute.js with appropriate responses/notifications