More binder

[bollwyvl]

https://gke2.mybinder.org/v2/gh/bollwyvl/accessibility/more-binder?urlpath=lab

• moves the repo specification to another yaml file
  • can't tell, but don't think the commits were checking out properly
• reworks entropy with a custom .yarn-links folder that doesn't break my local stuff 😊
• clobbers the main $PREFIX/share/jupyter/lab with stuff from ./jupyterlab/dev_mode
  • gives a shorter binder url... and means binder's "stock" federated extensions will load (e.g. widgets, offlineonotebook)
  • obviates need for custom start
• runs black

Trying to merge the current upstream HEAD was a bridge too far, and didn't work... but the machinery is there to do octopus-style merges, if they are all fast-forward. Behavior is somewhat undefined if the merges don't work.

As for validation, based on the two commits:

• the checkable command palette items have aria-checked set
• the sidebars don't say main sidebar navigation

So i think it's pulling stuff in, properly.