Previously, there was an `import services/webhooks` inside
`modules/notification/webhook`.
This import was removed (after fighting against many import cycles).
Additionally, `modules/notification/webhook` was moved to
`modules/webhook`,
and a few structs/constants were extracted from `models/webhooks` to
`modules/webhook`.
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
