POC Slack App bridging

[tadzik]

With RTM apps not being configurable anymore, and webhooks being terrible (and deprecated!), this is our best shot.

WIP required configuration:

Additional OAuth scopes needed:

• chat:write and chat:write.customize, for obvious reasons
• team:read required by clientFactory.upsertTeamByToken()
• users:read so that we can set profiles of Slack Ghosts
• files:read to be able to download attached files
• groups:read for Team Sync
• channels:join to be able to join Slack rooms when using Team Sync