Warn before assigning to many people

Problem to solve

There is no warning when a quick action assigns to many users.

Further details

Screenshot_2018-09-27_all_is_misaligned_in_autocompletion_dropdown___51929____Issues___GitLab_org_GitLab_Community_Edition

(please ignore the bar in the screenshot)

Screenshot_2018-09-27_all_is_misaligned_in_autocompletion_dropdown___51929____Issues___GitLab_org_GitLab_Community_Edition_1_

Proposal

Display a warning You are assigning to a lot of people.