Reviewers sidebar: Add unassign and invite members to new dropdown

...ipts/invite_members/components/invite_members_trigger.vue

...merge_requests/components/reviewers/reviewer_dropdown.vue

...vascripts/sidebar/components/reviewers/reviewer_title.vue

app/assets/javascripts/sidebar/mount_sidebar.js

app/assets/stylesheets/framework/sidebar.scss

app/helpers/issuables_helper.rb

app/views/shared/issuable/_sidebar.html.haml

ee/app/helpers/ee/issuables_helper.rb

ee/app/views/projects/quality/test_cases/show.html.haml

ee/spec/frontend/epic/components/epic_form_spec.js

...erge_requests/components/reviewers/approval_rules_spec.js

...lnerabilities/new_vulnerability/new_vulnerability_spec.js

locale/gitlab.pot

...e_requests/components/reviewers/reviewer_dropdown_spec.js

...ntend/sidebar/components/reviewers/reviewer_title_spec.js