Turtle page levels etc

Depends on #83

So the goal of this PR is to get Turtle in line with Maze & Karel where the levels aren't identified by PAGE and LEVEL numbers.

Turtle does a lot more checking of those variables for building up answers, blocks, etc, so those variables still exist and hence are parsed out of the level name split on underscores. We can specify them explicitly in the levels if we want to do that, so that we can rename and re-arrange, but this PR doesn't do that yet.

Lots more details in the commit message.