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/main/views/service_settings.py

@@ -38,6 +38,7 @@ from app.main.forms import (
     OrganisationTypeForm,
     FreeSMSAllowance,
     ServiceEditInboundNumberForm,
+    SMSPrefixForm,
 )
 from app import user_api_client, current_service, organisations_client, inbound_number_client
 from notifications_utils.formatters import formatted_list
@@ -103,7 +104,8 @@ def service_settings(service_id):
         default_letter_contact_block=default_letter_contact_block,
         letter_contact_details_count=letter_contact_details_count,
         default_sms_sender=default_sms_sender,
-        sms_sender_count=sms_sender_count
+        sms_sender_count=sms_sender_count,
+        prefix_sms_with_service_name=current_service['prefix_sms_with_service_name'],
     )
 
 
@@ -486,6 +488,28 @@ def service_set_sms(service_id):
     )
 
 
+@main.route("/services/<service_id>/service-settings/sms-prefix", methods=['GET', 'POST'])
+@login_required
+@user_has_permissions('manage_settings', admin_override=True)
+def service_set_sms_prefix(service_id):
+
+    form = SMSPrefixForm(enabled=(
+        'on' if current_service['prefix_sms_with_service_name'] else 'off'
+    ))
+
+    if form.validate_on_submit():
+        service_api_client.update_service(
+            current_service['id'],
+            prefix_sms_with_service_name=(form.enabled.data == 'on')
+        )
+        return redirect(url_for('.service_settings', service_id=service_id))
+
+    return render_template(
+        'views/service-settings/sms-prefix.html',
+        form=form
+    )
+
+
 @main.route("/services/<service_id>/service-settings/set-international-sms", methods=['GET', 'POST'])
 @login_required
 @user_has_permissions('manage_settings', admin_override=True)