[Front] Simple exercise user interface

[matthieusb]

This will be the page where a user practises an exercises. For this simple version, the user should be able to :

• Start/Stop the exercise
• Choose several exercises modes : show (alwarus answer hint), hide (always hide), delay (show with delay)
• In started mode : Type in the shortcuts and see if they were right or wrong
• See a list of previously typed shortcuts below the input line (With right/wrong showed with green or red color codes)