Fix "current value" for <b-select> element in e.g. edit form views

apparently marking an `<option>` as "selected" does not cut it for Buefy, and
we must bind to a v-model somehow.  not real crazy about the current method,
but it does seem to work okay so far...

tailbone/templates/forms/deform_buefy.mako

@@ -95,6 +95,8 @@
                       field_model_${field.name}: null,
                   % elif isinstance(field.schema.typ, deform.FileData):
                       field_model_${field.name}: null,
+                  % else:
+                      field_model_${field.name}: ${json.dumps(field.cstruct)|n},
                   % endif
               % endif
           % endfor