Add scripts/export-members.py

[NMertsch]

We used this utility during the volunteer onboarding at EP2024. It significantly simplified the identification of missing role assignments.

It runs a bot which exports the member lists of all accessible servers and then stops itself.

Usage:

$  export BOT_TOKEN="..."
$ python scripts/export-members.py member-lists/
$ tree member-lists/
member-lists
├── <GUILD-1-ID>-members.csv
├── ...
└── <GUILD-N-ID>-members.csv

Closes #194

[NMertsch]

Force-merged to move forward.

If you have feedback, please don't hesitate to reach out! :)