Add balance functions to stats command

[AlexHTW]

• added ADMIN_USER_IDS to .env, added is_admin function
• added user budget info to stats
• moved grant balance info (Add balance function #75) to stats: only shown when admin and has grant balance/time left
• added OpenAI billing info for current month (ex.: https://api.openai.com/dashboard/billing/usage?start_date=2023-03-01&end_date=2023-03-31) to stats: only shown for admin
• restructured usage_text for easier personalization of output

Could probably optimize a bit by calling get_remaining_budget in is_within_budget, not sure if the redundancy is too bad right now.

[AlexHTW]

The grant balance output could also include the expiration date, let me know if I should add it.

[AlexHTW]

Probably better to explicitly list admin(s) in the .env, will test and update PR soon.

[AlexHTW]

Added ADMIN_USER_IDS to .env, user ids here have the same access as ALLOWED_TELEGRAM_USER_IDS but without budget restrictions.
is_admin function can be used for future admin exclusive commands (like #108).
For now only used to add the OpenAI grant and billing info to admin's stats command.

[n3d1117]

Thanks @AlexHTW, looks good!

[AlexHTW]

Thanks for the merge and the new release @n3d1117 !
Could you please also credit @deanxizian in the release notes for this part?

[n3d1117]

Could you please also credit @deanxizian in the release notes for this part?

Done! Sorry, release notes were auto-generated