Add GitLab users to collaborators invite suggestions

Add GitLab users to collaborators invite suggestions

You can see the Get Started: Spread the word box by creating a room with no other users

Relevant code

• modules/collaborators/lib/collaborators-service.js
• modules/collaborators/test/collaborators-service-test.js
• scripts/utils/list-invite-user-suggestions.js
• server/api/private/invite-user-suggestions.js