Reword the add service page

Without the preview service name we probably have to go back to communicating
a bit more on the add service page. This commit brings back the two bullet
points about where the service name will appear, and tries to tie it into the
nice words that Matt Sheret wrote for us.

tests/app/main/test_add_service_form.py

@@ -9,4 +9,4 @@ def test_form_should_have_errors_when_duplicate_service_is_added(app_):
         form = AddServiceForm(_get_form_names,
                               formdata=MultiDict([('name', 'some service')]))
         form.validate()
-        assert {'name': ['Service name already exists']} == form.errors
+        assert {'name': ['This service name is already in use']} == form.errors