Feature Request: Mark Questions for Review

This feature would be intended to address scenarios like the following:

• You have solved the question, but your solution was not optimal and you want to come back later.
• Although you have solved the question this time, you are not 100% confident you could come up with the solution every time.

Currently, we may mark problems as complete (green) or incomplete (white). How about adding a third state called review (yellow), to mark problems that you can solve, but still are not 100% confident in?

I would be happy to make a PR for this if it's something we're interested in.