[Feature Request]: Duolingo/Anki-style spaced repetition review

[Marviel]

First off -- this is such an awesome project. I had a similar idea independently the other day, and rhapsodized about it to a colleague. Then, suddenly, I realized the idea was so obvious it could have been independently discovered. I googled it, and here we are. Fantastic work.

---- Back to regularly scheduled issue programming ----

"If you don't use it, you lose it" seems to be a property of the human mind, thus spaced repetition mechanisms like Anki decks have been found very useful for those needing to absorb large amounts of information.

The way metacademy is laid out, this could take several forms, but I figure a simple implementation would include:

• Concepts keep track of when they were "checked"
• A page wherein you can go through and "review" what you have learned, and provide "strength" to that particular concept, by demonstrating your knowledge of it, which would either reset the timer, or add to some "number of reviews" variable associated with that concept.

If any of this is unclear please let me know and I'll explain in more depth. I believe this is one improvement could make this a tool more usable by the general public, in more domains than are currently addressed.