Implement effective spaced repetition algorithm

[daniel-lerch]

Vocup currently chooses words to practice randomly with a certain distribution. That, however, is not very effective to practice a huge vocabulary book because words should be repeated a few times in the beginning and less often as mistakes get less but never disappear completely like they do now if the practice count is reached.

[Twometer]

Actually, many spaced repetition systems let words disappear completely - the idea being that you don't need to repeat the word for "Hello" 10 years into learning your target language. One of the most popular SRS examples, Anki, does this by setting the interval for the next repetition so absurdly high that it effectively "disappears" (100years).
Other examples include WaniKani, an SRS-based Japanese learning app, which has a separate SRS stage ("burned") for words that don't come back up for review - with the option to add them back into the lessons if the user so desires. Maybe something like this would be an option for Vocup?