Backport of multiple_assignees_feature [ci skip]

app/assets/javascripts/blob/target_branch_dropdown.js

app/assets/javascripts/blob/template_selector.js

app/assets/javascripts/boards/boards_bundle.js

app/assets/javascripts/boards/components/board_new_issue.js

app/assets/javascripts/boards/components/board_sidebar.js

app/assets/javascripts/boards/components/issue_card_inner.js

...assets/javascripts/boards/components/new_list_dropdown.js

app/assets/javascripts/boards/models/assignee.js

app/assets/javascripts/boards/models/issue.js

app/assets/javascripts/gl_dropdown.js

app/assets/javascripts/issuable/issuable_bundle.js

...pts/issuable/time_tracking/components/no_tracking_pane.js

...avascripts/issuable/time_tracking/time_tracking_bundle.js

app/assets/javascripts/issue_status_select.js

app/assets/javascripts/issues_bulk_assignment.js

app/assets/javascripts/labels_select.js

app/assets/javascripts/main.js

app/assets/javascripts/members.js

app/assets/javascripts/milestone_select.js

app/assets/javascripts/namespace_select.js