Improve support for sub-class snippets through the website

[GrahamTheCoder]

Converting just a method will appear to work, but in fact it's missing all symbol information and hence will make lots of wrong guesses about the conversion.

Currently there's a hack which catches total conversion failures and tries again with a method/class surrounding the code. The proper solution is to detect up front when we have a partial snippet, and surround it in a method and/or class as appropriate.

See example of code that doesn't work without context at #18