[Slack integration v2] Sync with Slack channel

[reichert621]

Description

This PR allows our users to sync a public Slack channel (e.g. a #support channel) with Papercups.

Example:

• Users links public #support channel to Papercups via the Sync with Slack integration
• Now, whenever a new message comes into the channel, a new conversation is spawned in Papercups
• The user can now reply to the message in multiple places:
  • In the Papercups dashboard
  • In their primary Slack channel (i.e. our Slack integration v1), which handles replying to Papercups messages via Slack
  • Or, of course, in the #support Slack channel itself -- in this case, replies will also be synced to the other channels above (both Papercups dashboard + Slack integration v1)

TODOs:

• Set up webhook handler for syncing with a Slack channel
• Update slack_authorizations table to support both types of integration
• Refactor Slack module to support both types of integration
• Update Integrations UI to support both types of integration
• Write more tests!

Screenshots

TODO: include gif of whole flow!

Checklist

• Everything passes when running mix test
• Ran mix format
• No frontend compilation warnings