[FIX] survey: Use subject from template in surveys

When a user updates the subject on a template used for a survey, it
does not pull the subject from the template. It uses the default
subject that is used when computing the subject for a survey email that
is sent out. This bug came as a part of [this commit].

Steps to reproduce on runbot:
1. Go to mail templates and search for Survey
2. Open the Survey: Invite template
3. Update the subject on this template
4. Open the Surveys app and select any survey
5. Click on Share and then enable send by email
6. The subject here will default to "Participate to {Survey Name}"

[this commit]: odoo@220e027

opw-4654411

closes odoo#211153

Signed-off-by: Romain Derie (rde) <rde@odoo.com>

Author: lecr-odoo

addons/survey/tests/test_survey_invite.py

@@ -139,6 +139,7 @@ def test_survey_invite(self):
             set([self.customer.email]))
         self.assertEqual(answers.mapped('partner_id'), self.customer)
         self.assertEqual(set(answers.mapped('deadline')), set([deadline]))
+        self.assertEqual(invite.subject, self.env["mail.template"].browse(action["context"]["default_template_id"]).subject)
 
         with self.subTest('Warning when inviting an already invited partner'):
             action = self.survey.action_send_survey()

addons/survey/wizard/survey_invite.py

@@ -154,9 +154,9 @@ def create(self, vals_list):
 
     @api.depends('template_id', 'partner_ids')
     def _compute_subject(self):
-        for invite in self:
-            if invite.subject:
-                continue
+        for invite in self.filtered(lambda inv: not inv.subject):
+            if invite.template_id and invite.template_id.subject:
+                invite.subject = invite.template_id.subject
             else:
                 invite.subject = _("Participate to %(survey_name)s", survey_name=invite.survey_id.display_name)