[IMP] project: usability of "Merge tasks" wizard

Display "Assigned to" field only if "Create a new task" checkbox is
checked (else it has no meaning)

addons/project/wizard/project_task_merge_wizard.py

@@ -7,7 +7,7 @@ class ProjectTaskMergeWizard(models.TransientModel):
     _name = 'project.task.merge.wizard'
 
     task_ids = fields.Many2many('project.task', string="Tasks to Merge", required=True)
-    user_id = fields.Many2one('res.users', string="Assigned to", required=True)
+    user_id = fields.Many2one('res.users', string="Assigned to")
     create_new_task = fields.Boolean('Create a new task')
     target_task_name = fields.Char('New task name')
     target_project_id = fields.Many2one('project.project', string="Target Project")

addons/project/wizard/project_task_merge_wizard_views.xml

@@ -8,7 +8,6 @@
                     <sheet>
                         <group>
                             <group>
-                                <field name="user_id"/>
                                 <field name="task_ids" widget="many2many_tags"/>
                             </group>
                             <group>
@@ -16,6 +15,7 @@
                                 <field name="target_task_name" attrs="{'invisible': [('create_new_task', '=', False)], 'required': [('create_new_task', '=', True)]}"/>
                                 <field name="target_project_id" attrs="{'invisible': [('create_new_task', '=', False)], 'required': [('create_new_task', '=', True)]}"/>
                                 <field name="target_task_id" attrs="{'invisible': [('create_new_task', '=', True)], 'required': [('create_new_task', '=', False)]}"/>
+                                <field name="user_id" attrs="{'invisible': [('create_new_task', '=', False)], 'required': [('create_new_task', '=', True)]}"/>
                             </group>
                         </group>
                         <span class="text-muted">NB: This will archive the selected tasks (Except the destination task)</span>