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

package.json

@@ -2,9 +2,16 @@
   "license": "MIT",
   "devDependencies": {
     "autoprefixer": "9.5.1",
+    "eslint": "5.16.0",
     "less": "3.9.0",
     "less-plugin-clean-css": "1.5.1",
     "lesshint": "^6.3.6",
-    "postcss-cli-simple": "3.0.0"
-  }
+    "postcss-cli": "6.1.2"
+  },
+  "browserslist": [
+    "> 1%",
+    "last 2 firefox versions",
+    "last 2 safari versions",
+    "ie 11"
+  ]
 }