Mentioned epics are linked to issues instead of epics

Summary

In the gitlab-dedicated group, when mentioning an MR in an epic, the comment in the MR about this mention links to an issue instead of the epic.

The current behavior can be observed in this MR.
The MR was mentioned in this epic, but the link in the MR points to this issue.

Steps to reproduce

1. in an epic inside the gitlab-dedicated group, mention any new created MR
2. in the mentioned MR, the note is displaying the correct text: xyz mentioned in epic gitlab-dedicated#11111111
3. but the actual link in the note points to the issue #11111111

Example Project

What is the current bug behavior?

The link in the note leads to an issue.

What is the expected correct behavior?

The link in the note leads to an epic.

Relevant logs and/or screenshots

Source

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

/cc @kushalpandya