MDL-40867 External tools should launch pop-ups from the course page

Without this fix, if an LTI tool is set to open in a pop-up, then
clicking the link on the course page opens another almost blank page,
which then tries to open the pop-up, which is almost invariably caught
by your pop-up blocker.

With this fix, clicking the link on the course page opens the pop-up
directly, like for pop-up resources.