Refactor the webhook logic, to have the type-dependent processing happen
only in one place.

---

## Current webhook flow

1. An event happens
2. It is pre-processed (depending on the webhook type) and its body is
added to a task queue
3. When the task is processed, some more logic (depending on the webhook
type as well) is applied to make an HTTP request

This means that webhook-type dependant logic is needed in step 2 and 3.
This is cumbersome and brittle to maintain.

Updated webhook flow with this PR:
1. An event happens
2. It is stored as-is and added to a task queue
3. When the task is processed, the event is processed (depending on the
webhook type) to make an HTTP request

So the only webhook-type dependent logic happens in one place (step 3)
which should be much more robust.

## Consequences of the refactor

- the raw event must be stored in the hooktask (until now, the
pre-processed body was stored)
- to ensure that previous hooktasks are correctly sent, a
`payload_version` is added (version 1: the body has already been
pre-process / version 2: the body is the raw event)

So future webhook additions will only have to deal with creating an
http.Request based on the raw event (no need to adjust the code in
multiple places, like currently).

Moreover since this processing happens when fetching from the task
queue, it ensures that the queuing of new events (upon a `git push` for
instance) does not get slowed down by a slow webhook.

As a concrete example, the PR #19307 for custom webhooks, should be
substantially smaller:
- no need to change `services/webhook/deliver.go` 
- minimal change in `services/webhook/webhook.go` (add the new webhook
to the map)
- no need to change all the individual webhook files (since with this
refactor the `*webhook_model.Webhook` is provided as argument)

Fixes #28088 
Fixes #28094

Added missing tests.

---------

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

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>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>