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: ![Bildschirmfoto_2019-02-07_um_10.47.19](/uploads/777f25319fbd62ce1c75dba163ae01b7/Bildschirmfoto_2019-02-07_um_10.47.19.png) ### What is the expected *correct* behavior? a message is displayed to highlight that no search is performed: ![Bildschirmfoto_2019-02-07_um_10.49.34](/uploads/22a6dbb80e29469f578a60f0f423fca2/Bildschirmfoto_2019-02-07_um_10.49.34.png)
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