Diff suggestion user callout is broken
Summary
Dismissing the "diff suggestion popover" user callout button is broken.
Steps to reproduce
Start a new comment in a diff
What is the current bug behavior?
A popover appears that tells you about the suggestions feature. Pressing "got it" issues a request to /-/user_callouts
that returns a 400 error with no content
What is the expected correct behavior?
The popover should disappear
Relevant logs and/or screenshots
Output of checks
This bug happens on GitLab.com
Possible fixes
Introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29473 ; I added https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29701 on top of that to fix a broken master