MDL-52127 grunt: be aware of third party paths
1) Parse thirdpartylibs.xml and generate an array of third party
file paths to use in grunt tasks
2) In the lint tasks, we filter third party files from being linted
3) We add a new task to generate ignore files - currently for eslint,
but will be potentially useful for other things in the future
4) Remove .eslintignore from source control
Why have the ability to generate a .eslintignore file? For tooling
integration - by having the eslintignore file people can use other
eslint tools without having to just use grunt (e.g. editor
integrations).
