Store `session` model on `_converse.state`

src/headless/shared/_converse.js

@@ -147,10 +147,11 @@ export class ConversePrivateGlobal extends EventEmitter(Object) {
     }
 
     initSession () {
-        this.session?.destroy();
-        this.session = new Model();
+        this.state.session?.destroy();
+        this.state.session = new Model();
 
         // XXX DEPRECATED
+        this.session = this.state.session;
         Object.assign(
             this, {
                 jid: undefined,