Let caller use string data for <tailbone-datepicker>

don't require a Date object, since callers thus far have not expected that

tailbone/templates/themes/butterball/field-components.mako

@@ -321,7 +321,7 @@
         template: '#tailbone-datepicker-template',
 
         props: {
-            modelValue: Date,
+            modelValue: [Date, String],
             disabled: Boolean,
         },
 
@@ -333,9 +333,7 @@
 
         watch: {
             modelValue(to, from) {
-                if (this.orugaValue != to) {
-                    this.orugaValue = to
-                }
+                this.orugaValue = this.parseDate(to)
             },
         },