Webhooks Form should not save if no trigger is selected

A user is currently able to create a webhook without selecting a trigger. We should require that at least one hook event is selected before saving the form. In the screenshot below, no trigger was chosen.

Steps to reproduce

• Navigate to a project or group that you have at least owner -level status with
• Navigate to settings > Webhooks > Add a webhook
• Add a url for the webhook but do not check any triggers in the form
• Save. You're redirected to the /hooks page and can see the webhook that was created.