User should be able to configure multiple webhooks in form settings

[PhilReinking]

To make working with webhooks easier, it should be possible to configure multiple webhooks for each form.

For now, there is only one webhook setting for each form.

• [x] user can add multiple webhooks to the same form
• [x] webhooks should also have a name for better organisation
• [x] provide API routes to add/delete webhooks