Make undeletable (rather than unmovable) blocks gray [finishes #71353760]

Undeletable blocks are now the ones that are gray rather than unmovable (of which we have no more).

See:

https://github.com/code-dot-org/dashboard/pull/698
https://github.com/code-dot-org/blockly/pull/601
https://github.com/code-dot-org/blockly-core/pull/85