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

**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.