Added ".perseus-interactive" class to API and threaded through to appropriate elements

Summary:
Mobile needs a special selector for determining what elements should intercept scratchpad
touches (and the criteria is not always the same as that used for the web with the ".above-scratchpad"
selector). Previously, we simply hard-coded a bunch of selectors in the mobile JavaScript; with this
solution, we'll have flexibility in the future, allowing us to change what's above or below the
mobile scratchpad with this selector.

Test Plan: See D11895.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11896