Group members autocomplete should adhere to domain whitelist

Overview

In https://gitlab.com/gitlab-org/gitlab-ee/issues/7297, we designed the ability to whitelist domains and not allow adding of members with emails outside of that domain pattern.

We present all members on the instance in the autocomplete when typing in a member name. When attempting to add a member to a group outside this pattern, we present an error. Instead, we should simply not show members that don't meet the domain whitelist's criteria.

Proposal

• If a domain whitelist is configured for a group, don't present members that fall outside this pattern in the member autocomplete when attempting to manually add a member to the list.