[FIX]Do not migrate account_id to allow reconciliation of existing bank ...

...statements.

This resolves issue #151. When account_id from account.bank.statement.line is migrated from 7 to 8, the reconciliation workflow in 8 uses the account_id to reconcile automatically, skipping the step where it is possible to make reconciliation adjustments. This also prevented closed bank statements from being adjusted.
Renaming the account_id field allows us to use the new reconciliation process for existing bank statements.

The help argument of account_bank_statement_line.account_id further clarifies this: "This technical field can be used at the statement line creation/import time in order to avoid the reconciliation process on it later on. The statement line will simply create a counterpart on this account".