fix(appsettings): settings take only the values the panel can use

The Settings page saved whatever was posted, and some lists offered
values the panel cannot use:

- Quota Debug could not be turned off: "False" is a non-empty text and
  counted as on.
- VM Clone Name Prefix showed as a True/False list, so no prefix could
  be typed, and touching it set the prefix to True.
- Disk SCSI Controller listed " lsilogic" with a space, never shown as
  chosen, and virtio-blk, which is no SCSI controller.
- VM Machine Type listed x86_64, an architecture; VM CD-ROM Device
  listed virtio, a bus a CD-ROM cannot use.
- Logs per Page and Disk Owner UID/GID are read with int(): a letter
  broke the logs page and VM create.

A migration fixes the lists and moves a value they no longer offer to
the default (the clone prefix keeps its text). A listed setting now
takes one of its choices, and every number setting a whole number in
its range, checked before it is saved.

appsettings/migrations/0020_fix_setting_choices.py

@@ -0,0 +1,32 @@
+from django.db import migrations
+
+# key: (its choices, the value a removed choice becomes)
+FIXES = {
+    "CLONE_INSTANCE_DEFAULT_PREFIX": ("", None),  # a text, not True or False
+    "INSTANCE_VOLUME_DEFAULT_SCSI_CONTROLLER": ("virtio-scsi,lsilogic", "virtio-scsi"),  # virtio-blk is no SCSI controller
+    "INSTANCE_MACHINE_DEFAULT_TYPE": ("q35,pc,virt", "q35"),  # x86_64 is an architecture
+    "INSTANCE_CDROM_ADD": ("None,ide,sata,scsi", "sata"),  # a CD-ROM has no virtio bus
+}
+
+
+def fix_choices(apps, schema_editor):
+    """Choices the panel cannot use, a space that kept lsilogic from being
+    shown as chosen, and a text setting listed as True or False. Not undone
+    on the way back, as nothing needs the old choices."""
+    AppSettings = apps.get_model("appsettings", "AppSettings")
+    for setting in AppSettings.objects.using(schema_editor.connection.alias).filter(key__in=FIXES):
+        choices, fallback = FIXES[setting.key]
+        setting.choices = choices
+        if choices and setting.value not in choices.split(","):
+            setting.value = fallback
+        setting.save()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("appsettings", "0019_prebuilt_themes"),
+    ]
+
+    operations = [
+        migrations.RunPython(fix_choices, migrations.RunPython.noop),
+    ]