analytics-activity: Fix UTC timezone and adjust header type.

Fixes an error with how the `utctime` was being generated and makes
the header on the summary table the same type as the other tabs.

analytics/views/installation_activity.py

@@ -294,7 +294,7 @@ def realm_summary_table(realm_minutes: Dict[str, float]) -> str:
         dict(
             rows=rows,
             num_active_sites=num_active_sites,
-            utctime=now.strftime("%Y-%m-%d %H:%MZ"),
+            utctime=now.strftime("%Y-%m-%d %H:%M %Z"),
             billing_enabled=settings.BILLING_ENABLED,
         ),
     )