Add module account_statement_import_base

The 2 modules account_statement_import_online and
account_statement_import depend on account_statement_import_base (and
not on each other) and share common code, in particular a hook to update
the statement line. So we can now have reconciliation modules that use
this hook and therefore work both on file import and online import. More
details on https://github.com/OCA/bank-statement-import/issues/481.

Improve bank statement line form view and journal form view.

account_statement_import/models/__init__.py

@@ -1,2 +1 @@
 from . import account_journal
-from . import account_bank_statement_line