Separate text and HTML content paths with new `Scribe.insertPlainText` method

This also adds [integration] tests for the `convertNewLinesToHtml` formatter.

I was hoping this would solve https://trello.com/c/EDAYdRBy/444-scribe-content-copied-from-firefox-and-then-pasted-into-composer-all-browsers-contains-random-line-breaks, but we can’t reproduce this issue anymore for some reason. Will revisit if needed, but this is a nice change to have nonetheless.