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
<details>
<summary>Expand for output related to GitLab environment info</summary>
<pre>
(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`)
</pre>
</details>
#### Results of GitLab application Check
<details>
<summary>Expand for output related to the GitLab application check</summary>
<pre>
(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)
</pre>
</details>
### Possible fixes
(If you can, link to the line of code that might be responsible for the problem)
issue