Added Spaced Repetition for Highlights

I'm taking my sweet time with Lurnby for sure, but I'm moving forward all the time. I have a lot of things that I want to change, but at the same time, I need to keep building out the functions I want so that I can learn sooner what works and what doesn't.

As always, I'm not positive this is the perfect experience, but I'm quite pleased that version 1 of spaced repetition for highlights now exists.

Definitely something I've looked forward to for a long time.