UADIGITAL-1255: Add automated linting to test coding standards.

Revives the earlier attempt to use ESLint for Javascript linting and stylelint for CSS, following the current settings in Drupal core. The current Drupal 8 JS recommendations are based on the Airbnb JavaScript Style Guide and ES6, but there is also a ESLint configuration file intended for older code, core/.eslintrc.legacy.json, which is directly copied here. The core/.stylelintrc.json CSS configuration differed too much from the current UA Bootstrap CSS to be immediately useful, but was the point of departure for a modified version here. The linting files are at the top level of the repository rather than in devtools/ subtree because they’re potentially useful to developers' editors and IDEs as well as to automated CI tools. More tweaking of the rules might be necessary, because they complain on many lines in many .js and .css files.