Prevent line segment from being dragged in copy-segment problem

Summary:
Avoid the sneaky solution of creating a line segment from A to B and then just dragging it to P.
For this sneaky person: https://khanacademy.atlassian.net/browse/AI-167076

Test Plan: Make sure line segment cannot be dragged on constructions_1.html?debug&problem=copy-segment, but can be on other problems

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D13425