Enable autocomplete for milestone descriptions

Summary

When using an autocomplete character in a milestone's description, the autocompletion does not take place.

Steps to reproduce

1. Create any item that could be referenced via an autocomplete character (I tried issues and milestones).
2. Go to issues > milestones > create new milestone
3. In the description box, use the autocomplete character for the item, you created in the first step, (I tried # and %).

Example Project

You should be able to try this in any project, even this one.

What is the current bug behavior?

Nothing.

What is the expected correct behavior?

A list of suggestions, similar to the behavior when creating a new issue and using an autocomplete character.

Output of checks

This bug happens on GitLab.com