[FIX] web: BasicModel: fetch added x2many records

Suppose that there is a many2many (list) field in a form view,
and there is a many2many (e.g. many2many_tags) field inside the
list.

In edit mode, if the user adds a record in the relation, this
record and its relational data should be fetched.

Before this rev., the relational data wasn't fetched, so the
many2many_tags couldn't be displayed correctly.