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>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Add support for triggering webhook notifications on wiki changes.

This PR contains frontend and backend for webhook notifications on wiki actions (create a new page, rename a page, edit a page and delete a page). The frontend got a new checkbox under the Custom Event -> Repository Events section. There is only one checkbox for create/edit/rename/delete actions, because it makes no sense to separate it and others like releases or packages follow the same schema.

![image](https://user-images.githubusercontent.com/121972/177018803-26851196-831f-4fde-9a4c-9e639b0e0d6b.png)

The actions itself are separated, so that different notifications will be executed (with the "action" field). All the webhook receivers implement the new interface method (Wiki) and the corresponding tests.

When implementing this, I encounter a little bug on editing a wiki page. Creating and editing a wiki page is technically the same action and will be handled by the ```updateWikiPage``` function. But the function need to know if it is a new wiki page or just a change. This distinction is done by the ```action``` parameter, but this will not be sent by the frontend (on form submit). This PR will fix this by adding the ```action``` parameter with the values ```_new``` or ```_edit```, which will be used by the ```updateWikiPage``` function.

I've done integration tests with matrix and gitea (http).

![image](https://user-images.githubusercontent.com/121972/177018795-eb5cdc01-9ba3-483e-a6b7-ed0e313a71fb.png)

Fix #16457

Signed-off-by: Aaron Fischer <mail@aaron-fischer.net>

* Added tests for MS Teams.

* Added tests for Dingtalk.

* Added tests for Telegram.

* Added tests for Feishu.

* Added tests for Discord.

* Added tests for closed issue and pullrequest comment.

* Added tests for Matrix.

* Trim all spaces.

* Added tests for Slack.

* Added JSONPayload tests.

* Added general tests.

* Replaced duplicated code.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>