Turn off linting for khan-exercises using a big hammer.

My recent change to delete khan-exercises from the codebase apparently
triggered us to start running things like javascript_lint.py against
khan-exercsies again.

Since khan-exercises is deprecated, and I'm removing it Very Soon, I
just turned off all linting in the repo.

Auditors: emily

Test Plan:
Ran `tools/runlint.py khan-exercises/` from webapp, without error.

lint_blacklist.txt

@@ -7,3 +7,5 @@ third_party/MathJax/2.1/config/*
 third_party/MathJax/2.1/extensions/*
 build/third_party/**
 test/**
+
+**