Add prune-alerts command

Combined with the new notifier queue in line#94 add a prune command so we
can avoid having our database grow too much

promgen/management/commands/prune-alerts.py

@@ -0,0 +1,34 @@
+# Copyright (c) 2018 LINE Corporation
+# These sources are released under the terms of the MIT license: see LICENSE
+
+import datetime
+
+from django.core.management.base import BaseCommand
+from django.utils import timezone
+from promgen import models
+
+
+class Command(BaseCommand):
+    def add_arguments(self, parser):
+        parser.add_argument('--days', type=int, default=30)
+        parser.add_argument('--force', dest='dryrun', action='store_false')
+
+    def success(self, message, *args):
+        self.stdout.write(self.style.SUCCESS(message % args))
+
+    def handle(self, days, dryrun, verbosity, **options):
+        cutoff = timezone.now() - datetime.timedelta(days=days)
+
+        if verbosity > 1:
+            self.success('Removing alerts before %s (%d days)', cutoff, days)
+
+        alerts = models.Alert.objects.filter(created__lt=cutoff)
+
+        if dryrun:
+            self.success('Would have removed %d alerts', alerts.count())
+            return
+
+        count, objects = alerts.delete()
+
+        if verbosity > 1:
+            self.success('Removed %d Alerts', count)