Remove or edit minimum account age for trustworthy

[mohammadrafigh]

Use case

There is a hardcoded 21 days minimum account age limit That I think there should be a way to modify it using configs. Actually it doesn't make sense to prevent users from contributing in exercises in a private instance where all users are trusted.

Proposal

There should be some config to modify it dynamically.

[mohammadrafigh]

I sent a PR here and in docker repo. The changes are minor but I'm not a Python developer so please review carefully.