Incorrect Milestones for filtering on Group Issues

Summary

Milestone filtering on Group Issues (e.g. https://gitlab.com/groups/gitlab-org/issues) shows milestones from dashboard issues. When a user is not logged in, they will get an error message.

Steps to reproduce

1. Open any Group Issues (e.g., https://gitlab.com/groups/gitlab-org/issues) on your browser (with larger screen to see the milestone filtering dropdown) and click the milestone filtering dropdown to get this bug.
2. Repeat the above step in an incognito windown

What is the current bug behavior?

for logged in user

Currently logged in users can see different milestones from targeted Group's ones when trying to filter Group Issues by Milestone on Web UI.

for anonymous user

Currently anonymous (non-login) users can see an unreasonable error message when trying to filter Group Issues by Milestone on Web UI.

Relevant logs and/or screenshots

for anonymous user

Output of checks / Results of GitLab environment info

This bug happens on GitLab.com. (as of GitLab EE 9.3.0-rc5)

/label ~bug