Code Review questions must contain code

As requested by Kaz and approved by the voters here on meta, questions asked on Code Review that do not contain code are now blocked.

I've configured this initially as a three-line minimum (or three separate bits of code-formatted text, or any combination of the two). This may be too much; it may more easily be too little. Happy to change it to anything else y'all decide is right.

If insufficient code is included in the question, a small error is shown below the editor:

This links to https://codereview.stackexchange.com/help/how-to-ask, which seems like a fairly comprehensive set of instructions on the matter.

As with the code formatting check and the code/explanation ratio check, this only applies to users at or below 200 reputation.

Please let me know if you have any questions or encounter any problems with this.