MDL-54944 eslint: make the .eslintrc default stricter

MDL-54944 eslint: make the .eslintrc default stricter

Instead of using the more rollup-tolerant config for all files, make the
configuration stricter by default but lower in grunt for yui modules.

This means that manual runs (prechecker) or editor integrations will
flag up the errors as well as grunt.

Also add ignore lines to the core files so that eslint isn't noisy when
not running with grunt.