Extract template / csv utility code into modules

This follows a similar approach to the previous commits, noting that
one module depends on the other, so we have to extract both together.

tests/app/utils/test_templates.py

@@ -0,0 +1,10 @@
+import pytest
+from notifications_utils.template import Template
+
+from app.utils.templates import get_sample_template
+
+
+@pytest.mark.parametrize("template_type", ["sms", "letter", "email"])
+def test_get_sample_template_returns_template(template_type):
+    template = get_sample_template(template_type)
+    assert isinstance(template, Template)