Added feature flag to show/hide assignees GraphQL widget [RUN ALL RSPEC] [RUN AS-IF-FOSS]

Merged Natalia Tepluhina requested to merge ntepluhina-assignees-feature-flag into master

What does this MR do?

Due to multiple comments about UX regression and search issues in #328004 (closed) (collaborators and members of invited groups are not show consistently), we will show an old jQuery dropdown by default while iteratively improving a GraphQL widget behind the feature flag.

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Related issue: #328004 (closed)

Edited by Natalia Tepluhina