With an upcoming bok-choy upgrade, 'not-present' and 'invisible' are two distinct concepts. In the upgrade, an invisible element must be present on the page. This commit draws that distinction across two helper methods, one for 'invisible' and the other for 'not-present' (or, not on the page at all).
common ------ This directory contains common code shared between LMS and CMS, such as Mako templates, CSS, and Coffescript.