Allow to unlink Monobank user from the system

[letehaha]

We need to allow users to unlink the whole Monobank account from the system. While doing that, all the accounts should be kept and not removed, so that the history will remain and it will be possible to link integration back.

Backend additional implementation details:

1. When unlinking, make all monobank accounts marked as system ones.
2. Store to account's addditionalData original monobank external accountId.
3. Keep old externalId so that in the future we can link them back.

Frontend additional details:

1. When unlinking monobank user, ask what to do with old linked accounts – remove or keep them. It should be a list of preselected checkboxes for each account. Keep checkbox selected will mean keep and unselected remove.

Also, when linking the Monobank account back, we need to compare if there were accounts that were related to that user earlier and ask the user to sync them. Otherwise, all statistics might be broken. Also, if the user entered some transactions manually to the account while it was unselected, we need to compare them with the existing transactions in history. Better will be to compare by date + amount, but might be good to compare by only amount. Also since all the Monobank transactions should be synced in 100% (there's no theoretical possibility of not having transactions that the user entered manually in Monobank DB), we might even mark unlinked transactions somehow specifically after the account linking, and ask the user to carefully review them to link. It can be implemented with that issue or in a separate one.