Slight reorganization in app.py and config.py, add nameservers env

Fix bug involving nonexistent users, remove unnecessary identifier conversions

Refactor to use user IDs for checking instead of usernames, fix notification duplication bug by storing processed_ids instead of newest_id, add load handling to AiohttpManager

Add mention flag, fix inconsistencies in some output messages

Handle API and server errors more gracefully, improve maintainability of codebase, bug fixes