Allow updates to SMS prefixing setting

We’re extracting this from being determined based on what the sender
name is to its own setting.

This commit will let users set it independently.

Until the explicitly set it, it will still be determined based on
whether their default sender name matches the default for the platform.

app/templates/views/service-settings.html

@@ -92,6 +92,12 @@
             {{ edit_field('Manage' if sms_sender_count else 'Change', url_for('.service_sms_senders', service_id=current_service.id)) }}
           {% endcall %}
 
+          {% call row() %}
+            {{ text_field('Start all text messages with service name') }}
+            {{ boolean_field(prefix_sms_with_service_name) }}
+            {{ edit_field('Change', url_for('.service_set_sms_prefix', service_id=current_service.id)) }}
+          {% endcall %}
+
           {% call row() %}
             {{ text_field('International text messages') }}
             {{ boolean_field('international_sms' in current_service.permissions) }}

app/templates/views/service-settings/sms-prefix.html

@@ -0,0 +1,24 @@
+{% extends "withnav_template.html" %}
+{% from "components/radios.html" import radios %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block service_page_title %}
+  Start all text messages with service name
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <h1 class="heading-large">Start all text messages with service name</h1>
+  <p>
+    Your service name is {{ current_service.name}}.
+  </p>
+  <form method="post">
+    {{ radios(form.enabled) }}
+    {{ page_footer(
+      button_text="Save",
+      back_link=url_for('.service_settings', service_id=current_service.id),
+      back_link_text='Back to settings'
+    ) }}
+  </form>
+
+{% endblock %}