Fixed answers not going through and progress being reverted in lessons, quizzes and exams

In chat-style lessons, answers that fail to send are now kept and resent before completion, with spacing between retries so they're less likely to fail again. We also fixed a just-finished exercise or quiz being marked unfinished when restarted right away.

In rank exams you can now keep selecting answers while the previous one saves, and your latest choice is the one saved. In quizzes, we fixed answers being accepted twice and responses from a previous question affecting the next one.

Lesson questions you can retry no longer reveal the correct answer when you get them wrong.