Add prompt to add first template to dashboard

https://www.pivotaltracker.com/story/show/112814667

> As you need to have templates to send any notifications, we should be nudging
> people to do that when they sign in.
>
> This should be in the dashboard, with a link to manage templates.
>
> Should be bright and shiny and only show if the service has no templates.

This commit adds the above.

It also rationalises the language (some places used ‘create template’, others
used ‘add template’, this changes everything to the latter).

app/templates/views/manage-templates.html

@@ -15,7 +15,7 @@ GOV.UK Notify | Manage templates
         <h1 class="heading-xlarge">Templates</h1>
 
         <p>
-          <a href="{{ url_for('.add_service_template', service_id=service_id) }}">Create new template</a>
+          <a href="{{ url_for('.add_service_template', service_id=service_id) }}">Add new template</a>
 
         {% for template in templates %}
           {% if template.get_field('template_type') == 'sms' %}

app/templates/views/service_dashboard.html

@@ -8,6 +8,15 @@
 
 {% block maincolumn_content %}
 
+    {% if not has_templates %}
+      {{ banner(
+        '<a href="{}">Add a template</a> to start sending notifications'.format(
+          url_for(".add_service_template", service_id=service_id)
+        )|safe,
+        type="tip"
+      )}}
+    {% endif %}
+
     <ul class="grid-row job-totals">
       <li class="column-half">
         {{ big_number(