Start porting to new urql api

It compiles. Not tested

web/frontend/src/PlotSelection.svelte

@@ -1,17 +1,21 @@
 <script>
     import { Modal, ModalBody, ModalHeader, ModalFooter, InputGroup,
              Button, ListGroup, ListGroupItem, Icon } from 'sveltestrap'
-    import { mutation } from '@urql/svelte'
+    import { gql, getContextClient , mutationStore } from '@urql/svelte'
 
     export let availableMetrics
     export let metricsInHistograms
     export let metricsInScatterplots
 
-    const updateConfigurationMutation = mutation({
-        query: `mutation($name: String!, $value: String!) {
+    const updateConfigurationMutation = ({ name, value }) => {
+    result = mutationStore({
+        client: getContextClient(),
+        query: gql`mutation($name: String!, $value: String!) {
             updateConfiguration(name: $name, value: $value)
-        }`
+        }`,
+        variables: { name, value }
     })
+    }
 
     let isHistogramConfigOpen = false, isScatterPlotConfigOpen = false
     let selectedMetric1 = null, selectedMetric2 = null