- add `make js`, deprecating `make javascripts`
- add `make css`, deprecating `make generate-stylesheets` and
`make stylesheets-check`
- changed the unclean css check to only run on CI
- add JS linting via eslint with basic configuration and fixed
discovered issues
- changed autoprefixer to use official `postcss-cli` avoiding the need
to loop in the makefile
- moved browserslist to package.json so other future tools can use it
too.
- update documentation for new make targets and added JS section
