Update utils to 12.0.0

Includes:

- [x] https://github.com/alphagov/notifications-utils/pull/94 (breaking
      changes which are responsible for all the changes to the API in
      this PR)

The test for `get_sms_fragment_count` has been removed because this
method is already tested in utils here:

ac20f7e99e/tests/test_base_template.py (L140-L159)

app/utils.py

@@ -1,4 +1,6 @@
 from flask import url_for
+from app.models import SMS_TYPE, EMAIL_TYPE
+from notifications_utils.template import SMSMessageTemplate, PlainTextEmailTemplate
 
 
 def pagination_links(pagination, endpoint, **kwargs):
@@ -18,3 +20,9 @@ def url_with_token(data, url, config):
     token = generate_token(data, config['SECRET_KEY'], config['DANGEROUS_SALT'])
     base_url = config['ADMIN_BASE_URL'] + url
     return base_url + token
+
+
+def get_template_instance(template, values):
+    return {
+        SMS_TYPE: SMSMessageTemplate, EMAIL_TYPE: PlainTextEmailTemplate
+    }[template['template_type']](template, values)