Summatives are no more!

With mastery challenges doing context switching for us, there's no longer a need for summative exercises that are just mixes of other exercises. In some cases we've removed the individual exercises from the site and left the summative. In other cases we've removed the summative and left the individual exercises. Thus I was able to inline all the remaining summatives without duplicating anything. TODO: get rid of the actual summative code.

Test Plan: Sanity check complementary_and_supplementary_angles, parallel_lines_1, parallel_lines_2, and combining_like_terms_2 in local mode. Also `grep 'div class="exercise" data-name' exercises/*` and verify that no live exercises are using summatives.

Auditors: alex