Get rid of the check for _hashed-* files; those files are gone gone gone.

Summary:
I deleted the code that produces them 2 months ago.

I also added support for the new third_party directory I want to create.

Test Plan: Fingers crossed.

Reviewers: alpert, eater

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D4424
Move third party things to third-party; get rid of d3

Reworked 2 exercises that used d3 in a really pointless way

Test plan: Test exercises in local mode: reading_bar_charts_1, reading_line_charts_1, addition_1, graphing_linear_equations, matrix_addition_and_subtraction. For each, verify exercises render, no errors in console, hints work, and correct answer is accepted.

Auditors: csilvers, alpert
third_party, not third-party

Test plan: Open a few exercises that use third_party things in local mode and verify no console errors.

Auditors: csilvers
Fix up the lint blacklist now that third-party code is in third_party/.

I also fixed some lint errors in a file I thought was third-party
code, but it turns out is actually by us.

Auditors: eater

Test plan:
This passes without error:
   khan-exercises% ~/khan/devtools/khan-linter/runlint.py .