Use bank's real balance instead of initialBalance + sum of transactions

• See the real balance for a real bank account (tested locally, not everyday in production)
• Resync transactions without breaking the balance for a real bank account
• Adding a future transaction, counting in ongoing balance for a real bank account
• Adding a past transaction, not modifying the balance for a real bank account
• Manual bank accounts should return the transactions sum as balance
• Add migration to remove balance from manual accounts
• Update the balance client-side for transaction creation on manual bank accounts
• Update the balance client-side for transaction deletion on manual bank accounts
• Update the balance client-side after transactions merge
• Demo bank accounts should return the transactions sum as balance
• Add migration to remove balance from Demo accounts
• Use current balance instead of initial balance for balance charts

Fix #1152 (closed) Fix #781 (closed)