Limit months shown to current and past

Matches what we do on the usage page. No need to see months in the
future because there’s no way you’ll have sent any messages in those
months, unless you’re Marty McFly.

app/main/views/dashboard.py

@@ -215,12 +215,17 @@ def format_monthly_stats_to_list(historical_stats):
     return sorted((
         dict(
             date=key,
-            name=datetime(int(key[0:4]), int(key[5:7]), 1).strftime('%B'),
+            future=YYYY_MM_to_datetime(key) > datetime.utcnow(),
+            name=YYYY_MM_to_datetime(key).strftime('%B'),
             **aggregate_status_types(value)
         ) for key, value in historical_stats.items()
     ), key=lambda x: x['date'])
 
 
+def YYYY_MM_to_datetime(string):
+    return datetime(int(string[0:4]), int(string[5:7]), 1)
+
+
 def aggregate_status_types(counts_dict):
     return get_dashboard_totals({
         '{}_counts'.format(message_type): {