add slack notifications for wiki pages

What does this MR do?

Lets the Slack service be configured to send notifications when wiki pages are created or edited.

Are there points in the code the reviewer needs to double check?

I'm just starting to get familiar with the Gitlab codebase and I was unsure on how to get the wiki page url to pass it to the slack message, on whether or not I needed to refactor the create/update methods for wiki pages from the controller to a service (but seemed necessary to test it better), and if I needed to add a column to the web hooks table or if the services table would have been enough. Please let me know if I should change anything and I will improve the MR, thanks for checking :)

Why was this MR needed?

Related to #563 (moved) and fixes #4233 (closed).