Trap focus inside modal/dialog

[jrobind]

When the lesson view modal appears there is no focus trap which is bad for assistive tech users. Currently, there is no way for a user to tab onto the close button and screen readers will also be unaware that a modal has appeared on screen.

I suggest we follow the W3 spec: https://www.w3.org/TR/wai-aria-practices-1.1/examples/dialog-modal/dialog