feat(templates): the shared form and delete pages in a centred column

The pages many views share were a horizontal form whose Cancel went
back in a primary colour beside a green Save, and a delete question in
a yellow alert squeezed into half the width, which wrapped word by word
on a phone.

Both now sit in a centred column with their heading: the form's fields
with their labels above in a card, Cancel outlined and Save the primary
action; the delete page asks "Delete <name>?" in a card with a red
edge, says what deleting does, and offers Cancel and Delete.

The form for a user's rights on a VM names them as the VM's Users tab
does: Change settings, Delete the VM, and Change console settings,
which only counts together with Change settings.

accounts/forms.py

@@ -32,6 +32,10 @@ class UserInstanceForm(ModelForm):
     class Meta:
         model = UserInstance
         fields = "__all__"
+        # the rights beyond power and console, as the VM's Users tab names them
+        labels = {"is_change": _("Change settings"), "is_delete": _("Delete the VM"),
+                  "is_vnc": _("Change console settings")}
+        help_texts = {"is_vnc": _("Only together with Change settings")}
 
 
 class UniqueEmailMixin: