Improve UX expectations for `contentEditable`

`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](#link)
- 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:

- https://github.com/fregante/GhostText/issues/53