Improve UX expectations for contentEditable

[fregante]

contentEditable is sent as raw HTML. This is usually expected (judging by the wording used in feature requests and bug reports)

At a minimum, there should be a (permanent?) notification explaining what's happening. I don't want to introduce a confirmation popup, even if it only appears once (because then that information would be lost)

Maybe this information should just be part of the connection popup:

• Connected to plain text field
• Connected to raw HTML field. More info
• Connected to CodeMirror field

The link would point to: https://ghosttext.fregante.com/troubleshooting/

Often contentEditable appears for unrecognized editors, so the help page should include this, perhaps suggesting to switch to the plain text field if one is available (#249, #219)

Also:

• html -> markdown -> html #53