Exclude account from stats

[letehaha]

There should be an option to exclude account from stats. It means that:

1. It won't affect balance.
2. Records of this account won't be visible in all statistics/listings.

It should be implemented on the account layer, not records of this account, to not override future isExcluded field that will point to that only specific transaction is excluded. We might want to reuse isEnabled which already hides the account from the dashboard. For now this field is only used for external accounts to optimize transactions loading, but we can extend the responsibilities of this field.