[C2] Multi-level attendance marking

[ywarnier]

Allow the attendance tool to give more than just "present/absent".
Ideally, this would be something flexible for the future (by allowing someone to configure the type of attendance), but otherwise it is just about introducing a little bit of nuance, because some scores can be established on the base of the number of attendances toward a total.

The types of attendances would be as follows (scores are tentative for now):

• [ ] absent (score: 0)
• [ x ] present (score: 1)
• [ - ] late less than 15 minutes (score: 1)
• [ + ] late more than 15 minutes (score: 0.5)
• [ v ] absent, justified (score: 0.25)

These scores are tentative (we'll have to try those out) but they are necessary as one can use the gradebook to score someone and deliver them a certificate.