Assign myself does not work from work item create form

Summary

Clicking "Assign myself" from the work item creation flow results in an empty assignee/not assigned.

Steps to reproduce

1. Start creating a work item via Epics > New epic (on gitlab-org or with :work_item_epics enabled)

Example Project

gitlab-org

What is the current bug behavior?

Not assigned

What is the expected correct behavior?

Assigned to current user

Relevant logs and/or screenshots

Output of checks

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