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