Define eslint rules for Closure Compiler externs

[Robbert]

We need to prevent unwanted errors for the extern definition files. So far I have discovered:

• we need to disable the environment variables, because nothing "exists" yet
• we need to disable warnings for unused variables, because this would trigger warnings for every argument of every dummy function declaration
• "use strict" makes no sense in these files (I suspect), disallow it.

Worries:

• there are dependencies between extern files, I suspect there might be a problem there