feat: Updates widget message state to support multiple conversations

app/javascript/widget/store/modules/message/getters.js

@@ -0,0 +1,7 @@
+export const getters = {
+  uIFlags: $state => $state.uiFlags,
+  messageById: _state => messageId => {
+    const message = _state.messages.byId[messageId];
+    return message;
+  },
+};