digest: send emails if you've been active for slightly less than a day.

That way if all you do is briefly check Zulip because you got the
email, we'll send you another one tomorrow.

(imported from commit fcbbd264c5e5fea7352f0fee6989e000af7b7bed)

zerver/management/commands/enqueue_digest_emails.py

@@ -38,7 +38,7 @@ def inactive_since(user_profile, cutoff):
     return last_visit < cutoff
 
 def last_business_day():
-    one_day = datetime.timedelta(hours=24)
+    one_day = datetime.timedelta(hours=23)
     previous_day = datetime.datetime.now(tz=pytz.utc) - one_day
     while previous_day.weekday() not in VALID_DIGEST_DAYS:
         previous_day -= one_day