Show warning when posttype template doesn't use replacement variables

[hedgefield]

Reported by @jdevalk: when someone enters copy in the Content Types template editor which doesn't include any replacement vars, that text goes out on every post, resulting in tons of duplicate descriptions.

Suggestion is to show a warning when someone does this.

Depending on how bad this is, we could also consider disabling the Save Changes button until the user fixes it? I'm a bit hesitant as this page is quite long so the connection between that error and the disabled button might not be immediately clear.

Thoughts?