[frontend/submissions] Automatic join of users information in CSV exports

[anthonygego]

Is your feature request related to a problem? Please describe. Currently if you download the results of the submissions page query, you only get the username as user identification. A manual join has to be performed with the "Users" CSV to get more insight (realname, email, ...).

Describe the solution you'd like It would be useful to perform the join automatically and serve a CSV file with the user info.