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/templates/appsettings.html

@@ -50,7 +50,7 @@
                     {% if setting.choices %}
                         <select class="form-select" name="{{ setting.key }}"  onchange="this.form.submit()" title="{% trans setting.description %}">
                             {% for choice in setting.choices_as_list %}
-                                <option {% if setting.value == choice %} selected {% endif %} value={{ choice }}>{% trans choice %}</option>
+                                <option {% if setting.value == choice %} selected {% endif %} value="{{ choice }}">{% trans choice %}</option>
                             {% endfor %}
                         </select>
                     {% else %}