[Feature Request] Make the embedded HackMD note editable

[tai271828]

Is your feature request related to a problem?

This feature request is the subsequent task of #177 and pycontw/pycon.tw#456 .

Describe the solution you'd like

Maybe the iframe could keep the cookie

Describe alternatives you've considered

Simply make the notes publicly editable to everyone

[mattwang44]

Tried out Chrome, Firefox, & Safari. This situation happens on Chrome and Safari, Firefox does not block the cookie.