implement a more robust caching strategy for Slack user- and channel-ids

[jeffreywescott]

From ch2142

Description

Slack uses their own internal IDs rather than human-friendly names for both channels and users, and they don’t provide an api to translate between the two.

Right now, we are caching the user and channel list (in memory) for 5 minutes. This will work until we have too many users / channels for that to be reasonable (e.g., 10s of thousands). We should fix this.

Proposal

• store Slack user ID in IDM with user
• store Slack channel ID somewhere in the DB for the channels we create (goals)

[jeffreywescott]

[Comment from Clubhouse user @jeffreywescott:] @prattsj: move to P4 GH?