Clear checked rows when refreshing async grid data

we don't want to accidentally "remember" checked rows which aren't currently
visible...  at least not yet / by default

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

@@ -53,6 +53,8 @@ let TailboneGrid = {
                 this.firstItem = data.first_item
                 this.lastItem = data.last_item
                 this.loading = false
+                // TODO: should "merge" checked results from server?
+                this.checkedRows = []
             })
             .catch((error) => {
                 this.data = []