Introduce private docs and flatpages preview (#26)

* Add support for private documents
* Fix migrations after merge
* Rebase fixtures
* Filter private decoder docs
* Enable preview of unpublished pages

ram/metadata/migrations/0013_decoderdocument_private.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.5 on 2023-10-06 19:48
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("metadata", "0012_alter_decoder_manufacturer_decoderdocument"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="decoderdocument",
+            name="private",
+            field=models.BooleanField(default=False),
+        ),
+    ]