[FIX] gamification: allow employee to grant badge

If the user does not have the rights to write on the user profile, he will
trigger an access error.
Put it as a simple computed readonly field to avoid access errors

In 10.0, a workaround was developed at 0312c604f4 but it should be reverted
after this commit

addons/hr_gamification/wizard/gamification_badge_user_wizard.py

@@ -9,7 +9,8 @@ class GamificationBadgeUserWizard(models.TransientModel):
     _inherit = 'gamification.badge.user.wizard'
 
     employee_id = fields.Many2one('hr.employee', string='Employee', required=True)
-    user_id = fields.Many2one('res.users', string='User', related='employee_id.user_id', store=True)
+    user_id = fields.Many2one('res.users', string='User', related='employee_id.user_id',
+        store=False, readonly=True)
 
     @api.multi
     def action_grant_badge(self):