Ref #6463, #6428
* pin isort to 5.6.4
* update isort version mentioned in python_guidelines.rst
* make lint commands consistent between make lint and circleci
* fix isort errors on files outside of /wagtail
* speed up isort by ignoring additional directories and filetypes
Running `npm install` without `--no-save` can introduce spurious changes to package-lock.json due to differences in platform / node version, and these have been known to accidentally sneak into people's PRs. `npm install` without `--no-save` should only be used when actively updating node dependencies in package.json.
This installs wagtail as editable package in your current python env
including testing and docs extras. It also installs runs npm install
and generates the frontend files.
