remote_server: Migrate RemoteZulipServer.uuid to be UUIDField.

Given that these values are uuids, it's better to use UUIDField which is
meant for exactly that, rather than an arbitrary CharField.

This requires modifying some tests to use valid uuids.

zilencer/migrations/0021_alter_remotezulipserver_uuid.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.9 on 2021-12-22 10:06
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zilencer", "0020_remotezulipserverauditlog"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="remotezulipserver",
+            name="uuid",
+            field=models.UUIDField(unique=True),
+        ),
+    ]