binder Frontmatter field is ignored by thebe

[agoose77]

Right now, it's not easy to specify a custom mybinder URL, e.g. the obvious:

version: 1
project:
  binder: https://binder.opensci.2i2c.cloud/
  github: https://github.com/2i2c-org/demo-icepyx

Instead, we have to write:

version: 1
project:
  jupyter:
    binder:
      repo: https://github.com/2i2c-org/demo-icepyx
      url: https://binder.opensci.2i2c.cloud/

  github: https://github.com/2i2c-org/demo-icepyx

This seems to be because we don't listen (at all?) to the project.binder Frontmatter entry for anything.

[stevejpurves]

yes, correct I think we should remove that. Historically, way back(TM), project.binder was being used to display a classic [launch | binder] badge in the frontmatter, prior to even thebe being integrated IIRC. The theme's don't support it so we should drop it.