This commit adds a basic eslintrc that emulates jslint defaults.
Rules that conflict with our existing code have been switched to
warnings instead of errors. Globals have been added to the eslintrc. The
bundled js file (generated by webpack) and blueslip.js are ignored with
.eslintignore.
To display warnings, run npm run lint-loud. This runs eslint without the
--quiet option on static/js and frontend_tests.
npm run --silent lint is run by tools/lint-all (in addition to jslint).
The --silent option is used to suppress the default output from npm run.
Fixes#535.