Slack allows configuration of both incoming webhooks and bot tokens at the same time

[grobinson-grafana]

The Slack receiver allows configuration of both incoming webhooks and bot tokens at the same time. This is a bug. The intended behaviour is that users configure one or the other, but not both. The UI prevents users from being able to set both, but I think it's still possible when using the Provisioning API.

There are a number of incorrect tests that also assert both incoming webhooks and bot tokens can be configured at the same time. This is incorrect and should be fixed too.