Give admins option to test if a theme loads correctly

If a theme is uploaded incorrectly or has errors in it, users can still
select the theme but it will cause a 500 error on every page, making the
app unusable and also making it impossible for them to switch to a
functional theme.

A better fix would be to fail gracefully, but in lieu of that, this will
at least let admins confirm if a theme is broken safely.

bookwyrm/migrations/0187_theme_loads.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.23 on 2023-11-20 17:44
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("bookwyrm", "0186_invite_request_notification"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="theme",
+            name="loads",
+            field=models.BooleanField(blank=True, null=True),
+        ),
+    ]