Exempt files_controller#show_relative from protect_from_forgery
Exempt files_controller#show_relative from protect_from_forgery

Opened https://github.com/instructure/canvas-lms/pull/945 to pull into upstream.

grahamb/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms