Add export for user admin groups

Add export for user admin groups

• Refactor export resource to use native iterables so we can re-use existing functions that fetch the admin groups and return an array

Part of https://gitlab.com/gitlab-org/gitter/webapp/-/issues/1899

Dev notes

• http://thecodebarbarian.com/async-generator-functions-in-javascript.html
• https://thecodebarbarian.com/cursors-in-mongoose-45
• https://mongoosejs.com/docs/4.x/docs/api.html#cursor-QueryCursor-js
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of

Testing strategy

1. Set yourself as a staff user, mongo gitter -> db.users.update({ username: 'yourusername' }, { staff: true })
2. Visit your local Gitter, open the devtools and run window.troupeContext.user.id to get your user ID
3. Replace the user ID in the following URL
4. Visit http://localhost:5000/api_web/export/user/5a87396f1543b98772a686a3/admin-groups.ndjson and download your messages