Add invite option to reviewer dropdown [RUN AS-IF-FOSS]

...javascripts/invite_members/init_invite_members_trigger.js

app/assets/javascripts/pages/projects/issues/show.js

.../pages/projects/merge_requests/init_merge_request_show.js

app/assets/javascripts/sidebar/mount_sidebar.js

app/assets/javascripts/sidebar/track_invite_members.js

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

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

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

...pp/assets/javascripts/pages/projects/issues/show/index.js

...s/javascripts/pages/projects/merge_requests/show/index.js

spec/features/issues/issue_sidebar_spec.rb

...atures/merge_request/user_edits_assignees_sidebar_spec.rb

...atures/merge_request/user_edits_reviewers_sidebar_spec.rb

spec/frontend/sidebar/track_invite_members_spec.js

...mples/features/issuable_invite_members_shared_examples.rb