Rename ‘Send a message’ to ‘Templates’ in basic view

From Karl:

> Templates – this should be consistent with Admin view. Users may
> switch from Basic to Admin view (or vice versa), they will also
> interact with users who have a different view or permissions to them.
> Neither should have to learn new interfaces and language if possible.

> ‘Send a message’ was a nice, active label – but Notify options aren’t
> usually actions. If we’re going to change this we should be consistent
> across both Admin and Basic views.

> For the same reason, I have rejected ‘see’, ‘search’ and ‘view sent
> messages’. It will be interesting to see in user testing whether users
> read ‘sent messages’ as ‘send messages’.

app/templates/main_nav.html

@@ -20,7 +20,7 @@
 {% else %}
 <nav class="navigation">
   <ul>
-    <li><a href="{{ url_for('.choose_template', service_id=current_service.id) }}" {{ casework_navigation.is_selected('send-one-off') }}>Send a message</a></li>
+    <li><a href="{{ url_for('.choose_template', service_id=current_service.id) }}" {{ casework_navigation.is_selected('send-one-off') }}>Templates</a></li>
   </ul>
   <ul>
     <li><a href="{{ url_for('.view_notifications', service_id=current_service.id, status='sending,delivered,failed') }}" {{ casework_navigation.is_selected('sent-messages') }}>Sent messages</a></li>