add usernames to export file

[Hagellach37]

when downloading the users.csv file for a project you currently only get the user id, but no information on the user names. This would be nice to be able to display stats such as these on the mapswipe website.

This is addressed here in the code:

• https://github.com/mapswipe/python-mapswipe-workers/blob/master/mapswipe_workers/mapswipe_workers/generate_stats/user_stats.py
• we need to implement another function to actually get the user names from the postgres database