Add checkmarks to selected items in approval rule settings modal

Context

As part of the UX Scorecard (gitlab-design#441 (closed)), a first usability test around GitLab's Merge Request page showed that the approval rules can be difficult to understand. We followed up with a usability testing study (ux-research#270 (closed)) to investigate this further and uncover new insights.

Problem

Once you click into the "Approvers" field, you cannot see your selected items, in the approval rule settings modal. Being able to see both the dropdown and your already-selected items would be useful, to help you remember what you've already selected.

Proposal

Add checkmarks to selected items in the approval rule settings modal.

Screenshot

https://gitlab.com/gitlab-org/uxr_insights/uploads/5bf0d2e198c6949d89b1198efd09ba30/Screen_Shot_2019-12-16_at_3.22.07_PM.png

Edited by Katherine Okpara