activity: Show each realm's creation date.

I'd rather have something rich and fancy like a sparkline of
activity... but this is a lot quicker to implement.

analytics/views.py

@@ -287,6 +287,7 @@ def realm_summary_table(realm_minutes: Dict[str, float]) -> str:
     query = '''
         SELECT
             realm.string_id,
+            to_char(realm.date_created, 'YYYY-MM-DD') date_created,
             coalesce(user_counts.dau_count, 0) dau_count,
             coalesce(wau_counts.wau_count, 0) wau_count,
             (

templates/analytics/realm_summary_table.html

@@ -14,6 +14,7 @@
     <thead class="activity_head">
         <tr>
             <th>Realm</th>
+            <th>Created</th>
             <th>DAU</th>
             <th>WAU</th>
             <th>Total users</th>
@@ -33,6 +34,10 @@
                 {{ row.string_id }}
             </td>
 
+            <td>
+                {{ row.date_created }}
+            </td>
+
             <td class="number">
                 {{ row.dau_count }}
             </td>