Add ability to export user messages

Add ability to export user messages. This is currently limited to staff users to make sure the load is okay on our servers

Part of gitlab-com/www-gitlab-com#7652 (closed) -> https://gitlab.com/gitlab-org/gitter/webapp/-/issues/1899

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Restore some of the export infrastructure we had from Gitter topics. Based off of:

• https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1199
• https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1238

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/messages.ndjson and download your messages