Exercise logic: system should not know the correct answer

[erpreciso]

• [X] After all students answered, teacher should be able to validate or not the answer
  • [ ] show graph as hystogram
  • [ ] bars should be clickable from the teacher to validate the answer
• [X] in the exercise list there should not be any "valid" answer to be used from the process
  • [X] ideal exercise list is a list of sentences
• [X] always add an "other" option

[erpreciso]

except of the graphic part, all requests have been satisfied