zilencer: Tie RemotePushDeviceToken to RemoteRealm at registration.

This consists of the following pieces:
1. Makes servers using the bouncer send realm_uuid in requests for token
   registration. (Sidenote: realm_uuid is already sent in the "send
   notification" codepath as of
   48db4bf854)
2. This allows the bouncer to tie RemotePushDeviceToken to the
   RemoteRealm with matching realm_uuid at registration time.
3. Introduce handling of some potential weird edge cases around the
   realm_uuid and RemoteRealm objects in get_remote_realm_helper.

zilencer/migrations/0043_remotepushdevicetoken_remote_realm.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.2.7 on 2023-12-01 15:52
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zilencer", "0042_alter_remoterealmauditlog_realm_id"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="remotepushdevicetoken",
+            name="remote_realm",
+            field=models.ForeignKey(
+                null=True, on_delete=django.db.models.deletion.SET_NULL, to="zilencer.remoterealm"
+            ),
+        ),
+    ]