Don’t have service model inherit from dict

Inheriting from `dict` has some unexpected side effects that don’t
happen with plain object. The one we want to avoid right now is that
a dict doesn’t seem to implement `__dict__` in a normal way, which
is required by `werkzeug.utils.cached_property`.

app/templates/partials/check/too-many-messages.html

@@ -7,7 +7,7 @@
 </h1>
 <p>
   You can only send {{ current_service.message_limit }} messages per day
-  {%- if current_service.restricted %}
+  {%- if current_service.trial_mode %}
     in <a href="{{ url_for('.using_notify')}}#trial-mode">trial mode</a>
   {%- endif -%}
   .