[Popup Window]Replace the difficulty bar by understanding measurement

----update 14 feb 2020 User story: A user doesn't understand the old difficulty bar, because they get confused like: are you asking me the difficulty of this resource in general or just for me? what's the principle to judge it? They wish to see a clear instruction like: "How well do you understand this?" and also with options of 5 fix points. 3 levels are too less.

As the team has discussed, we are going to replace the difficulty bar by mastery bar.

the question to the user will change from "how difficult is this resource" to "How well do you understand it?"