[FIX] event_sale: workaround for broken view.

Non-stored fields created with the new API are not converted to old
fields and added in `_columns` (and thus `_all_columns`). This forbid
them to be used in qweb views.
A fix will be commited in 8.0 to address this issue.
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64