[IMP]Made 'Add an item' text bold for many2many widget

Purpose : 
'Add an item' text in many2many widget should be bold to maintain a standard aesthetic in odoo.
It is happened because class for one2many is added in base.css twise. so replace one of them with class for many2many. See the task.

Task : 
https://www.odoo.com/web#id=9024&view_type=form&model=project.task&menu_id=3942&action=327