Move data-ensures to vars div

PSA: Having a data-ensure around vars without id tags (like in `<div class="solution"><var>HEIGHT_B</var></div>`) is a no-go since they don't get reevaluated the second time around.

Test Plan:
solving_similar_triangles_2.html?debug&problem=height-b&seed=63 accepts 75/7, as it should.

Auditors: eater