Move dropdowns above scratchpad (z-index)

Summary: Give the dropdows the `interactive-component` class so they appear above the scratchpad and thus are interactive while the scratchpad is enabled. Fixes T3042.

Test Plan: Try both types of dropdowns with the scratchpad enabled (crossed fingers)

Reviewers: aria

Reviewed By: aria

Maniphest Tasks: T3042

Differential Revision: http://phabricator.khanacademy.org/D13172