Add flag to close a bank account

[mshiner]

Over time the list of bank accounts can become quite long. It would be useful to add a closed flag to a bank account. The account would then be excluded from all drop downs and so no further transactions can be added.

Issues for implementation

1. The account should have a zero balance
2. All cash book transactions should be reconciled
3. No active pl and sl accounts that have the accounts as default (i.e. inactive flag = F)
4. The default account flag should be false

Checks on transaction entry will need a backstop to prevent posting if closed = F (perhaps if a closed SL account is reactivated and the bank account change missed).

Can be implemented at the same time as issue #131 is fixed.

[stale]

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