1578 - Adds a unique destroy? policy for file version membership

[jlandiseigsti]

closes #1578

The fileVersionMembershipPolicy used to just delegate to the destroy policy of the WorkVersionPolicy. However, we want a slight change – while we don't want to be able to delete the non-recent work versions, we want to be able to delete files even if they are associated with older versions.

[ajkiessl]

This looks good. I did notice that the flash message set in the controller for deleting a file is 'Work version was successfully destroyed'. Since we're making changes in the area, we might as well fix this to say something like 'File was successfully destroyed'.

scholarsphere/app/controllers/dashboard/file_version_memberships_controller.rb

Lines 30 to 38 in f4419ca

	authorize(@file_version)
	@file_version.destroy
	respond_to do |format|
	format.html do
	redirect_to dashboard_form_files_path(@work_version),
	notice: 'Work version was successfully destroyed.'
	end
	format.json { head :no_content }
	end

[ajkiessl]

LGTM