analytics: Change messages_sent_to_stream to a daily stat.

Analytics database tables are getting big, and so we're likely moving to a
model where ~all stats are day stats, and we keep hourly stats only for the
last N days.

Also changed the name because:
* messages_sent_* suggests the counts (summed over subgroup) should be the
  same as the other messages_sent stats, but they are different (these don't
  include PMs).
* messages_sent_by_stream:is_bot:day is longer than 32 characters, the max
  allowable length for a BaseCount.property.

Includes a database migration to remove the old stat from the analytics
tables.

analytics/migrations/0009_remove_messages_to_stream_stat.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from django.db import migrations
+
+from analytics.lib.counts import do_delete_count_stat
+
+def delete_messages_sent_to_stream_stat(apps, schema_editor):
+    # type: (StateApps, DatabaseSchemaEditor) -> None
+    do_delete_count_stat('messages_sent_to_stream:is_bot')
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('analytics', '0008_add_count_indexes'),
+    ]
+
+    operations = [
+        migrations.RunPython(delete_messages_sent_to_stream_stat),
+    ]