Warn if the generated preview HTML is different from the cached HTML when starting to edit a post

Occasionally, the Markdown engine has breaking changes made to it; three such changes are coming up when CommonMark compatibility is brought in. There's a suggestion there for warning editors that things could break and semi-auto-updating posts. But it occurs to me that we could do a better job of that: any time the current engine's preview differs from the cached HTML when the post hasn't yet been edited at all, warn the editor accordingly (and, internally, provide a hook for semi-auto-conversion when appropriate?). This would handle all such deliberate changes in the future on a case-by-case basis with no further coding work needed, while still allowing for the occasional regex sweep of all posts for certain endemic patterns to fix (if that's ever judged practical). It would also help catch any bugs that creep in a little sooner.