Fix missing to on reassign MR email to unassigned.

Factors out MR and issue email.

app/views/notify/_reassigned_issuable_email.html.haml

+%p
+  Assignee changed
+  - if @previous_assignee
+    from
+    %strong #{@previous_assignee.name}
+  to
+  - if issuable.assignee_id
+    %strong #{issuable.assignee_name}
+  - else
+    %strong Unassigned

app/views/notify/reassigned_issue_email.html.haml

-%p
-  Assignee changed
-  - if @previous_assignee
-    from
-    %strong #{@previous_assignee.name}
-  to
-  - if @issue.assignee_id
-    %strong #{@issue.assignee_name}
-  - else
-    %strong Unassigned
-
+= render 'reassigned_issuable_email', issuable: @issue

app/views/notify/reassigned_merge_request_email.html.haml

-%p
-  Assignee changed
-  - if @previous_assignee
-    from
-    %strong #{@previous_assignee.name}
-  to
-  %strong #{@merge_request.assignee_name}
+= render 'reassigned_issuable_email', issuable: @merge_request