replace subscription form with htmx implementation

static/static/js/ext/json-enc.js

@@ -0,0 +1,12 @@
+htmx.defineExtension('json-enc', {
+    onEvent: function (name, evt) {
+        if (name === "htmx:configRequest") {
+            evt.detail.headers['Content-Type'] = "application/json";
+        }
+    },
+    
+    encodeParameters : function(xhr, parameters, elt) {
+        xhr.overrideMimeType('text/json');
+        return (JSON.stringify(parameters));
+    }
+});