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 -%}
   .

app/templates/views/service-settings.html

@@ -253,7 +253,7 @@
       {% endcall %}
     </div>
 
-    {% if current_service.restricted %}
+    {% if current_service.trial_mode %}
       <h2 class="heading-medium">Your service is in trial mode</h2>
 
       <ul class='list list-bullet'>
@@ -334,7 +334,7 @@
       <ul>
         <li class="bottom-gutter">
           <a href="{{ url_for('.service_switch_live', service_id=current_service.id) }}" class="button">
-            {{ 'Make service live' if current_service.restricted else 'Revert service to trial mode' }}
+            {{ 'Make service live' if current_service.trial_mode else 'Revert service to trial mode' }}
           </a>
         </li>
         <li class="bottom-gutter">