feat: Improvements ux

[dewita]

Improvements

• New scrollable area limited in height for text editor which prevents the user to be lost writing the exercise.
• The rich text editor buttons contain indications when hovering them.
• Change the behavior of an exercise when no rating has been made yet (a '-' replace the 0) in the preview of an exercise.
• Add highlighting when a link is active in the Menu.

Removed

• The overlay when overing the rating of the exercise for the preview of an exercise.