Add an ability to unlink and link back external account

[letehaha]

Change type to system when unlinking but store it's externalType in the additionalData so when user wants to link it back, we will have all the info.

Changes needed:

1. UI. Link/unlink buttons, handle linking back if no monobank user exists (just say we cannot link, firstly link your account)
2. Backend. New externalType to additionalData. When linking back through creating a new mono user, we need to make sure that account will have same accountId.
3. Add tests