Quick actions don't send any webhooks

Summary

When a quick action is triggered on a merge request, such as /assign, or adding WIP to the title, no webhook event is sent, as far as I can tell. To add support for GitLab to https://github.com/jenkins-x/lighthouse, I ideally need to be able to manage responses to /approve and WIP in title specifically, and it'd be handy to be notified in general when any other quick actions fire so that I can build responses to them.

Steps to reproduce

Listen for webhooks on a project, create a merge request, perform a quick action on it, don't receive a webhook. =)

Example Project

n/a

What is the current bug behavior?

No webhook is sent, even though a comment has been submitted or the merge request title has been changed.

What is the expected correct behavior?

A comment webhook or a merge request webhook, as appropriate, should be sent.

Relevant logs and/or screenshots

n/a

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)