Fix build, make statusbar and palette optional, update readme

[krassowski]

• [x] fixes eslint errors and warnings on CI
• [x] aligns with the current jupyterlab extension cookiecutter:
  • [x] adds CI and binder badge from cookiecutter
  • [x] updates eslint rules
  • [x] adds federated extension
• [x] makes statusbar and palette optional to make it possible to use with other frontends such as jupyterlab-classic
• [x] updates references to organization and repo to point to jupyterlab-contrib
• [x] updates outdated README
• [x] adds "Choose language" command to the command palette
• [ ] maybe rename to @jupyterlab-contrib/spellchecker? (will be done in the next PR to update extension under current namespace with links to this repository)

[krassowski]

All good, ready for review.

[ocordes]

All good, ready for review.

BTW, one question because I have not so much experiences in the work-flow of the pull requests in github. For such a merge it is better to combine all commits into one?

[krassowski]

Yes, squash is a good option here.