fix invitation emails for non-contributor referees

scipost_django/templates/email/referees/confirmation_invitation_response.html

-<p>Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},</p>
+<p>
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %},
+</p>
 
 <p>
   We hereby confirm your choice to {% if invitation.accepted %}accept{% else %}decline (due to reason: {{ invitation.get_refusal_reason_display }}){% endif %} to referee Submission

scipost_django/templates/email/referees/inform_referee_manuscript_withdrawn.html

 <p>
-  Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %}
 </p>
+
 <p>
   This is a simple email to inform you that the authors of
 </p>

scipost_django/templates/email/referees/invite_contributor_to_referee.html

-<p>Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},</p>
+<p>
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %},
+</p>
 <p>
   We have received a Submission to SciPost which, in view of your expertise and On behalf of the Editor-in-charge {{ invitation.submission.editor_in_charge.profile.get_title_display }} {{ invitation.submission.editor_in_charge.user.first_name }} {{ invitation.submission.editor_in_charge.user.last_name }}, we would like to invite you to referee:
   <br />

scipost_django/templates/email/referees/invite_contributor_to_referee_reminder1.html

 <h3>Re: refereeing invitation. First automatic reminder</h3>
-<p>Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},</p>
+
+<p>
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %},
+</p>
+
 <p>
   Recently, On behalf of the Editor-in-charge {{ invitation.submission.editor_in_charge.profile.get_title_display }} {{ invitation.submission.editor_in_charge.user.first_name }} {{ invitation.submission.editor_in_charge.user.last_name }}, we invited you to consider refereeing a Submission to {{ invitation.submission.submitted_to }}, namely
   <br />

scipost_django/templates/email/referees/invite_contributor_to_referee_reminder2.html

 <h3>Re: refereeing invitation. Second (and last) automatic reminder</h3>
-<p>Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},</p>
+
+<p>
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %},
+</p>
+
 <p>
   Recently, On behalf of the Editor-in-charge {{ invitation.submission.editor_in_charge.profile.get_title_display }} {{ invitation.submission.editor_in_charge.user.first_name }} {{ invitation.submission.editor_in_charge.user.last_name }}, we invited you to consider refereeing a Submission to {{ invitation.submission.submitted_to }}, namely
   <br />

scipost_django/templates/email/referees/remind_referee_deadline_1week.html

 <p>
-  Dear {{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }},
+  Dear {% if invitation.referee %}{{ invitation.referee.profile.get_title_display }} {{ invitation.referee.user.last_name }}{% else %}{{ invitation.profile.get_title_display }} {{ invitation.profile.last_name }}{% endif %},
 </p>
 <p>
   This is a simple email to remind you of the approaching deadline (in one week) to send a Report on