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.

instances/utils.py

@@ -62,7 +62,7 @@ def check_user_quota(user, instance, cpu, memory, disk_size):
     if instance <= 0 and cpu <= 0 and memory <= 0 and disk_size <= 0:
         return msg
 
-    quota_debug = app_settings.QUOTA_DEBUG
+    quota_debug = app_settings.QUOTA_DEBUG == "True"
 
     user_instances = UserInstance.objects.filter(user=user, instance__is_template=False).select_related(
         "instance__compute"