Disable 'Assign myself' button if already assigned

Closes: https://todo.sr.ht/~sircmpwn/todo.sr.ht/275

todosrht/templates/ticket.html

@@ -182,6 +182,10 @@
                 <button
                   name="myself"
                   class="btn btn-default"
+                  {% if current_user in ticket.assigned_users %}
+                  title="you are already assigned to this ticket"
+                  disabled
+                  {% endif %}
                 >Assign myself {{ icon('caret-right') }}</button>
                 <button class="btn btn-primary">
                   Assign {{ icon('caret-right') }}