Add print_phonehome management command

useful for:

* figuring out what phonehome will report
* getting some stats on-screen locally

phonehome/management/commands/print_phonehome.py

@@ -0,0 +1,12 @@
+import json
+from django.core.management.base import BaseCommand
+
+from phonehome.tasks import _make_message_body
+
+
+class Command(BaseCommand):
+    help = "Print the phonehome message to the console."
+
+    def handle(self, *args, **options):
+        message_body = _make_message_body()
+        print(json.dumps(message_body, indent=4))