Display requirement to type at least 3 characters when searching users

Summary

When typing user names in the author or assignee dropdown in issue search, nothing is displayed for one or two characters. Instead we should display a message.

This emerged from https://gitlab.com/gitlab-org/gitlab-ce/issues/38664#note_54015204.

Steps to reproduce

• go to issue list (https://gitlab.com/gitlab-org/gitlab-ce/issues)
• type author: wi)

What is the current bug behavior?

no message is displayed:

What is the expected correct behavior?

a message is displayed to highlight that no search is performed: