feat: Add slack participants sync mechanism

[spalmurray]

WIP. Adds a sync mechanism for slack participants on Incidents. Details:

• Sync happens on request to UI/Service API(todo) view.
• Throttled to at most one update per 5 minutes.
• If slack users do not have firetower accounts, will create one and sync it to IAP later on first login.
• Sync ignores bot users
• Adds a manual django admin action to sync also.
• Adds an endpoint to force a slack sync (will be triggered by refresh button in UI).

todos:

• rebase onto Service API and ensure urls and views are set up nicely.
• rebase onto ruff/mypy and add types

[keeakita]

I think this is a fine initial approach, but I think ideally I'd rather have Slack user information be its own model that is then JOINed to IAP user info, rather than updating the email field. It seems conceptually easier to work with as we won't have two types of user crammed into one data model