set 0'd dict in redis if no data exists in redis

to get the data for a day can be reasonably slow (a few hundred
milliseconds), and if someone's viewing a service with no activity we
don't want to do that query seven times every two seconds. So if there
is no data in redis, when we get the data out of the database, we
should put it in redis so we can just grab it from there next time.

This'll happen in two cases:
* redis data is deleted
* the service sent no messages that day

additionally, make sure that we convert nicely from redis' return
values (ascii strings) to unicode keys and integer counts.

app/template_statistics/rest.py

@@ -1,7 +1,8 @@
 from flask import (
     Blueprint,
     jsonify,
-    request
+    request,
+    current_app
 )
 
 from app import redis_store
@@ -61,15 +62,28 @@ def get_template_statistics_for_last_n_days(service_id, limit_days):
     template_stats_by_id = Counter()
 
     for day in last_n_days(limit_days):
+        print('\n')
         # "{SERVICE_ID}-template-usage-{YYYY-MM-DD}"
         key = cache_key_for_service_template_usage_per_day(service_id, day)
         stats = redis_store.get_all_from_hash(key)
-        if not stats:
+        if stats:
+            stats = {
+                k.decode('utf-8'): int(v) for k, v in stats.items()
+            }
+        else:
             # key didn't exist (or redis was down) - lets populate from DB.
             stats = {
                 str(row.id): row.count for row in dao_get_template_usage(service_id, day=day)
             }
-
+            # if there is data in db, but not in redis - lets put it in redis so we don't have to do
+            # this calc again next time. If there isn't any data, we can't put it in redis.
+            # Zero length hashes aren't a thing in redis. (There'll only be no data if the service has no templates)
+            if stats:
+                redis_store.set_hash_and_expire(
+                    key,
+                    stats,
+                    current_app.config['EXPIRE_CACHE_EIGHT_DAYS']
+                )
         template_stats_by_id += Counter(stats)
 
     # attach count from stats to name/type/etc from database
@@ -83,4 +97,7 @@ def get_template_statistics_for_last_n_days(service_id, limit_days):
             'is_precompiled_letter': template.is_precompiled_letter
         }
         for template in template_details
+        # we don't want to return templates with no count to the front-end,
+        # but they're returned from the DB and might be put in redis like that (if there was no data that day)
+        if template_stats_by_id[str(template.id)] != 0
     ]