Replace jquery.postmessage with vanilla postmessage implementation

[gunnarlium]

The 3rd party postmessage lib is designed to work with very old browsers that we no longer support, meaning it does a lot of unnecessary work. It also intercepts postmessage from other pages, which may not follow the format it expects. An example of this is when the data is already a JSON object, which does not need to be parsed.

Supporting vanilla postmessage is pretty straightforward in modern browsers.