Switch to eslint for linting

(cherry picked from commit a18c3774661f466727ab46315211aecb43ef1def)

Closes #3350

frontend/build/webpack/css-variables-loader.js

@@ -1,3 +1,4 @@
+// eslint-disable-next-line filenames/match-exported
 const loaderUtils = require('loader-utils');
 
 module.exports = function cssVariablesLoader(source) {

frontend/postcss.config.js

@@ -28,4 +28,4 @@ module.exports = {
     'postcss-color-function',
     'postcss-nested'
   ]
-};
+};