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

.eslintrc

@@ -0,0 +1,25 @@
+root: true
+
+extends:
+  - eslint:recommended
+
+parserOptions:
+  ecmaVersion: 2015
+
+env:
+  browser: true
+  jquery: true
+  es6: true
+
+globals:
+  Clipboard: false
+  CodeMirror: false
+  emojify: false
+  SimpleMDE: false
+  Vue: false
+  Dropzone: false
+  u2fApi: false
+  hljs: false
+
+rules:
+  no-unused-vars: [error, {args: all, argsIgnorePattern: ^_, varsIgnorePattern: ^_, ignoreRestSiblings: true}]