analytics: Add graphs of usage statistics on /stats.

This adds a frontend for the analytics system we've had for a few
months, showing several graphs of the data in Zulip.

There's a ton more that we can do with this tooling, but this initial
version is enough to provide users with a pretty good experience.

Fixes #2052.

analytics/views.py

@@ -63,7 +63,7 @@ def get_chart_data(request, user_profile, chart_name=REQ(),
         subgroups = ['false', 'true']
         labels = ['human', 'bot']
         include_empty_subgroups = True
-    elif chart_name == 'messages_sent_by_humans_and_bots':
+    elif chart_name == 'messages_sent_over_time':
         stat = COUNT_STATS['messages_sent:is_bot:hour']
         tables = [RealmCount]
         subgroups = ['false', 'true']