Managing access requests

[pawamoy]

1. user sends /start to the bot
2. the bot can't find the user in the DB and says, "sorry, you don't have access, send /requestAccess and wait"
3. user sends /requestAccess to the bot
4. bot says "thank you, I notified the admins" and sends a message to the admins, initiating a conversation to choose whether to accept or reject the request
5. admin accepts the request: user is created in the DB with the default permissions / admin rejects the request: nothing is done
6. user receives a notification saying they have been granted/denied access to the bot

The access requests could also be deactivated. In that case, we stop at step 2 and the bot just says "sorry".

To simplify admins work, we could store the last 100 users who sent /start and didn't have access with their usernames and ids in a file, so an admin can just send /grantAccess username to the bot, which would pick the right ID in the file.