Fix datepicker behavior for grid filters

apparently we *do* need to accept a 'value' prop for tailbone-datepicker, to
round out the v-model support

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

@@ -11,6 +11,7 @@ const TailboneDatepicker = {
         'icon="calendar-alt"',
         ':date-formatter="formatDate"',
         ':date-parser="parseDate"',
+        ':value="value ? parseDate(value) : null"',
         '@input="dateChanged"',
         '>',
         '</b-datepicker>'
@@ -19,6 +20,7 @@ const TailboneDatepicker = {
     props: {
         name: String,
         id: String,
+        value: String
     },
 
     methods: {