Enhance webhook integration UX

[Twixes]

Changes

Extracting some out-of-scope improvements from #2111, this modernizes and improves webhook integration UX – closes PostHog/posthog.com#564 and should also resolve #2203.
Also renames Team field slack_incoming_webhook to more general incoming_webhook.

Checklist

• Django backend tests (if this PR affects the backend).
• Cypress end-to-end tests (if this PR affects the frontend).

[mariusandra]

I think this is a solid refactor, however I do have to add (and I feel I'm repeating myself), that any alter/change/delete migration will result in downtime for users. Same here. We're in a situation where thousands of users of varying scale depend on posthog and we must minimise disruptions to service of any of them.

I know this feels dirty, but the best way forward here is either 1) not rename the field in the db and perhaps even wait until we move webhooks into plugins, 2) create a new field, incoming_webhook, copy over all the data and then just deprecate the old field without removing it.

Eventually when we release posthog 2.0, we can remove all the deprecations.

[Twixes]

Hm, yep, that's true, that field does also appear in tasks, so I removed the migration – it was an unimportant change anyway.