add make targets for js and css, add js linter (#6952)

* add make targets for js,css, add javascript linter

- 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

* fix indentation

* move functions used in html to 'exported' list

* Run lessc binary without having to install anything to node_modules

* use relative paths to node bin scripts, removing npx

* Revert "use relative paths to node bin scripts, removing npx"

This reverts commit 119b725525.

* fix lessc and postcss plugins

* check for node_modules and use actual bin names

.drone.yml

@@ -50,7 +50,8 @@ pipeline:
     pull: true
     commands:
       - npm install
-      - make stylesheets-check
+      - make css
+      - make js
     when:
       event: [ push, tag, pull_request ]