bug/investigation: required blocks are not automatically selected in the Level Editor

[evemartin]

Describe the bug
It is possible to create levels in the Level Editor without the required blocks to solve them, making the created level impossible. For example, it is possible to create levels with cows or traffic lights without the cow/traffic light-related blocks, and it is possible to create a level with a turn but remove the turn-related blocks.

To Reproduce
Steps to reproduce the behaviour:

1. Go to the Level Editor
2. Create a level with a turn in it
3. Deselect the "turn left" and "turn right" blocks in the Code tab
4. Save the level. Notice you are allowed to save the level even though it is impossible to solve

Expected behaviour
It should not be possible to save the level without the required blocks, i.e. every created level should be solvable.

Additional context
This task might need a longer-term, more investigative conversation first to determine what should be required to make a particular level solvable, and what cases this task should cover. Some cases are more straightforward (when there are cows/traffic lights those blocks should be required, when there is a turn in the road the turn blocks should be required, when there are multiple houses the deliver block should be required, etc.) but others might be more complicated