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.
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).
Fix#16457
Aaron F
