User autocomplete doesn't work with less than 3 characters anymore

Summary

Assignee input minimum length is increased to 3 characters.

Steps to reproduce

Click "Assignee", enter 1 or 2 characters. No results. Enter the next, enjoy the results.

What is the current bug behavior?

It only works with 3 or more characters

What is the expected correct behavior?

Work with 1 characters, as it used to work. It doesn't make any sense having it increased to 3 characters, we have 10 developers using GitLab. We're running GitLab 10

Alternatively, we should continue to display the list of relevant users for the scope even when 1 or 2 characters are entered, as discussed in https://gitlab.com/gitlab-org/gitlab-ee/issues/3782#note_43759149. We could couple this with a message to indicate that no filtering is happening and then start filtering on 3 characters.