Add a new statistics/analytics framework.

This is a first pass at building a framework for collecting various
stats about realms, users, streams, etc. Includes:
* New analytics tables for storing counts data
* Raw SQL queries for pulling data from zerver/models.py tables
* Aggregation functions for aggregating hourly stats into daily stats, and
  aggregating user/stream level stats into realm level stats
* A management command for pulling the data

Note that counts.py was added to the linter exclude list due to errors
around %%s.

scripts/lib/pythonrc.py

@@ -3,6 +3,7 @@ try:
     from django.conf import settings
     from zerver.models import *
     from zerver.lib.actions import * # type: ignore # Otherwise have duplicate imports with previous line
+    from analytics.models import *
 except Exception:
     import traceback
     print("\nException importing Zulip core modules on startup!")