Add 'disabled' prop for buefy datepicker

also make sure we return `null` when input is empty

tailbone/static/js/tailbone.buefy.datepicker.js

@@ -13,6 +13,7 @@ const TailboneDatepicker = {
         ':date-parser="parseDate"',
         ':value="value ? parseDate(value) : null"',
         '@input="dateChanged"',
+        ':disabled="disabled"',
         '>',
         '</b-datepicker>'
     ].join(' '),
@@ -20,12 +21,16 @@ const TailboneDatepicker = {
     props: {
         name: String,
         id: String,
-        value: String
+        value: String,
+        disabled: Boolean,
     },
 
     methods: {
 
         formatDate(date) {
+            if (date === null) {
+                return null
+            }
             // just need to convert to simple ISO date format here, seems
             // like there should be a more obvious way to do that?
             var year = date.getFullYear()