disable user exports by default

- new setting to enable user exports defaults to False
- add setting to enable and disable user exports
- do not allow user exports when using s3 storage
- do not serve non-image files from /images/ (requires update to nginx settings)
- increase default file upload limit to 100MB to enable user exports to be imported (can be changed in .env)

bookwyrm/migrations/0192_sitesettings_user_exports_enabled.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.23 on 2024-01-16 10:28
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("bookwyrm", "0191_merge_20240102_0326"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="sitesettings",
+            name="user_exports_enabled",
+            field=models.BooleanField(default=False),
+        ),
+    ]