Support private notes in Slack integration

[Twixes]

Problem
Private notes added in #521 are great – they are however not sendable from the Slack integration, nor are they viewable there.

Solution
There should be an explicit way to send a private note from Slack (e.g. the message is considered a note when if it begins with a slash), and when a private note is sent from somewhere else (i.e. the web app), it should in turn be displayed in the Slack thread.

Additional context
A use case which would make this specifically useful is mentioning teammates (other agents) inside Slack, e.g. for assistance, without notifying the customer.

Testing
Not sure how you test the Slack integration, but basically sending a message with in Slack and verifying that it is then saved as a note (and NOT as a reply) in Papercups would cover this end-to-end.

[reichert621]

@Twixes I did a little research into this, and it looks like Slack doesn't support slash commands in reply threads for some reason 😞 (see "Limitations on use" in https://api.slack.com/interactivity/slash-commands#enabling-interactivity-with-slash-commands__what-are-slash-commands)

So I'm thinking there are a couple alternatives to how we can handle this:

1. Any messages that @mentions the Papercups app/bot is considered a note
2. Any message that starts with [note] or 🔒 or ;; or some other agreed upon string/emoji is considered a note
3. Something else?

Curious to hear your thoughts/preference

[Twixes]

I think 1. could occasionally backfire if someone wants to do this intentionally in a normal reply. 2. sounds pretty good, just that the prefix should be simple to enter (definitely not an emoji) and practically never used in an online conversation/support cases at the start of the message. I was thinking a backslash would work well.

[reichert621]

backslash sounds good for now 👍

[reichert621]

done in #562!