Channel purge

[agarzola]

This PR implements two standalone cron jobs and one new endpoint in the Express server. It also establishes a sqlite3 database, with an interface via knex.js.

Notify channels job

Finds all unarchived channels with no activity in the past 30 days and:

• posts a message in the channel notifying users about intent to archive
• provides a cancellation button that removes the channel from the purge queue
• pins said message
• stores the channel’s id and the resulting message’s timestamp in a new row of the purge_queue table

Purge channels job

Finds all records in the purge_queue table and:

• archives the channel using each record’s channel_id value
• deletes the row from the purge_queue table

Cancel channel purge endpoint

Purge notification messages are interactive. They have a Don’t Archive button which, when clicked, triggers a POST request to this endpoint, which finds any records with the relevant channel’s ID in the purge_queue table and deletes them. If successful, the message is updated to reflect the fact that the channel is no longer in the purge queue, and unpins it. If unsuccessful, it returns a 500 status, which leaves the message intact and results in an ephemeral error message that only the user who clicked on the button can see.

[agarzola]

Need to add migrations to the deployment script.

[agarzola]

Gotta get this through next hacknight, srsly.

Also, we need to look into running the pm2 config as a service so that service interruptions don’t require a manual restart.

[brb3]

All looks good to me :+1:. I'm not a node expert by any stretch, but it looks like you are handling errors, bulding queries sanely, all that jazz. Merge that ish.