[FIX] web: many2many_binary widget upload

once widget extended with ReinitializeFieldMixin, the event binding with the binary file input and the on_file_change method can be done in initialize_content instead of start

This fix is related to d36c8b5c9bf7ab9215bc63db6ee49bec5e5b225b
[MERGE] forward port of branch 7.0 up to ae99a93