add new redis template usage per day key

We've run into issues with redis expiring keys while we try and write
to them - short lived redis TTLs aren't really sustainable for keys
where we mutate the state. Template usage is a hash contained in redis
where we increment a count keyed by template_id each time a message is
sent for that template. But if the key expires, hincrby (redis command
for incrementing a value in a hash) will re-create an empty hash.

This is no good, as we need the hash to be populated with the last
seven days worth of data, which we then increment further. We can't
tell whether the hincrby created the key, so a different approach
entirely was needed:

* New redis key: <service_id>-template-usage-<YYYY-MM-DD>. Note: This
  YYYY-MM-DD is BTC time so it lines up nicely with ft_billing table
* Incremented to from process_notification - if it doesn't exist yet,
  it'll be created then.
* Expiry set to 8 days every time it's incremented to.

Then, at read time, we'll just read the last eight days of keys from
Redis, and sum them up. This works because we're only ever incrementing
from that one place - never setting wholesale, never recreating the
data from scratch. So we know that if the data is in redis, then it is
good and accurate data.

One thing we *don't* know and *cannot* reason about is what no key in
redis means. It could be either of:

* This is the first message that the service has sent today.
* The key was deleted from redis for some reason.

Since we set the TTL to so long, we'll never be writing to a key that
previously expired. But if there is a redis (or operator) error and the
key is deleted, then we'll have bad data - after any data loss we'll
have to rebuild the data.

app/utils.py

@@ -79,6 +79,10 @@ def cache_key_for_service_template_counter(service_id, limit_days=7):
     return "{}-template-counter-limit-{}-days".format(service_id, limit_days)
 
 
+def cache_key_for_service_template_usage_per_day(service_id, datetime):
+    return "{}-template-usage-{}".format(service_id, datetime.date().isoformat())
+
+
 def get_public_notify_type_text(notify_type, plural=False):
     from app.models import SMS_TYPE
     notify_type_text = notify_type