Context: see parent ticket.

The following scenario needs communicated in the UI:

1. Admin adds Webhook "myWebhook" to config
2. User adds "myWebhook" to an alert
3. Admin removes "myWebhook" from the config

AT:

• the UI informs the user of webhooks that no longer exist in the config by displaying a warning sign next to the alert in the alerts list, which indluces a tooltip describing that the previously configured webhook is now no longer available
• in the alerts modal a warning is displayed at the top (similar to the missing email config warning) telling the user about the now missing webhook
• the webhook selection is cleared, so that when the user closes the alert again the alert is saved with no webhook associated with it