[Finishes #72141728] Support non-unique (Blockly) level names in custom scripts
replace deprecated first_or_create method.
use validating finder methods instead of explicit raise.
Merge pull request #736 from code-dot-org/blockly_levels_in_custom_script

[Delivers #72141728] Support non-unique (Blockly) level names in custom scripts
flappy k1 levels
Automatically built.
Merge pull request #738 from code-dot-org/flappyK1

flappy k1 levels
Merge pull request #739 from code-dot-org/finished

deploy to delivered