Cache returned letter summary in Redis

We’re going to start using the returned letters summary to show some
info on the dashboard.

This means we will be accessing it more often than it changes. And we
know exactly when it changes because it’s us manually submitting the
references we get from DVLA.

This makes it a good candidate for being cached, and Redis is where we
cache stuff that we’d otherwise go to the API for.

app/main/views/platform_admin.py

@@ -360,6 +360,9 @@ def platform_admin_returned_letters():
 
         try:
             letter_jobs_client.submit_returned_letters(references)
+            redis_client.delete_cache_keys_by_pattern(
+                'service-????????-????-????-????-????????????-returned-letters-summary'
+            )
         except HTTPError as error:
             if error.status_code == 400:
                 error_references = [