Address duplicated code in LevelView

[joshlory]

See this note in commit https://github.com/code-dot-org/craft/pull/476/commits/0c742c9dc1560370128b96d9ccd0ebf6b3d8247e:

A lot of duplication here in LevelView#createBlock, planning to address this in a follow-up PR.

We should also clean up the this.blocks = { ... } definition, down to just a mapping of block name to atlas frame (with optional offset override, or bake the new offsets into Blocks.json).