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).

tests/app/main/views/test_dashboard.py

@@ -4,6 +4,7 @@ from flask import url_for, session
 def test_should_show_recent_jobs_on_dashboard(app_,
                                               api_user_active,
                                               mock_get_service,
+                                              mock_get_service_templates,
                                               mock_get_user,
                                               mock_get_user_by_email,
                                               mock_login):

tests/app/main/views/test_sign_out.py

@@ -17,6 +17,7 @@ def test_sign_out_user(app_,
                        api_user_active,
                        mock_get_user,
                        mock_get_user_by_email,
+                       mock_get_service_templates,
                        mock_login):
     with app_.test_request_context():
         email = 'valid@example.gov.uk'