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 and fixes#4233.
See merge request !2998
Rails' "url_helpers" method creates an anonymous Module (which a bunch
of methods) on every call. By caching the output of this method in a
dedicated method we can shave off about 10 seconds of loading time for
an issue with around 200 comments.
Add new service classes to create and update project and personal
snippets. These classes are responsible for enforcing restricted
visibility settings for non-admin users.
Supports four different event types all bundled under the "note" event type:
- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet
Rémy CoutableandYorick Peterse
Robert Speicher
Dmitriy Zaporozhets
Stan Hu
Douwe Maan
Vinnie Okada
Ciro Santilli
Jeroen van Baarsen