feature: flashcard rating to influence the frequency in which a phrase is shown

[apttx]

i know a few characters and words very well, and others seem to never appear. it's be nice if i could tell the app to show me a phrase more or less often before going to the next flashcard.

• rate more or less
  • give every flashcard a score, one of
    • never show (never show on /flashcards)
    • show rarely
    • show normally (= initial value, once every iteration of the collection)
    • show often
    • show every other time (iterate over a fixed set of phrases [with random order] before moving on to a random one, then iterate over them again)
• show rating at the bottom left (opposite of flashcard controls)
• allow changing the rating on the collection page