Delete the old zulip.com "referrals" system.

This system hasn't been in active use for several years, and had some 
problems with it's design.  So it makes sense to just remove it to declutter
the codebase.

Fixes #5655.

zerver/migrations/0088_remove_referral_and_invites.py

@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.2 on 2017-07-07 08:34
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0087_remove_old_scheduled_jobs'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='referral',
+            name='user_profile',
+        ),
+        migrations.RemoveField(
+            model_name='userprofile',
+            name='invites_granted',
+        ),
+        migrations.RemoveField(
+            model_name='userprofile',
+            name='invites_used',
+        ),
+        migrations.DeleteModel(
+            name='Referral',
+        ),
+    ]