Mark Lessons as "Studied"

[ShahriarKh]

As a signed-in user, I wish to be able to mark a lesson as "studied" or "have read", which serves two purposes: 1. it's a motivation for me to read all the lessons and "check them all off" 2. help me organize which lessons have been read, especially when I jump to different topics without following any orders.

It could be implemented in multiple ways, for example:

add a button for user to "mark this lesson as read"
mark a lesson as read when user clicks "next lesson"
mark a lesson as read when user's browser tab has focused on this tab for more than x minutes
These behaviors could also be configurable as user preference.

Depending on the implementation, a lesson could even be read multiple times, for example, a ✔ is shown on the page to indicate it's been read, and ✔✔✔ shows if it's been read for 3 times. Clicking or hovering the symbol could show the date it happened, and the duration of the session, etc.

See the discussion #1913.