Let a grid have custom ajax data url

i.e. instead of assuming we should GET the current "partial" page, can use a
different URL altogether now

tailbone/static/js/tailbone.buefy.grid.js

@@ -46,7 +46,7 @@ let TailboneGrid = {
             }
 
             this.loading = true
-            this.$http.get(`?${params}`).then(({ data }) => {
+            this.$http.get(`${this.ajaxDataUrl}?${params}`).then(({ data }) => {
                 this.data = data.data
                 this.rowStatusMap = data.row_status_map
                 this.total = data.total_items