Handle feedback on courses.

Backend

⚠ Review/update the backend structure for this matter. ⚠

Currently we have the following architecture:

Several notes:

• One possible design to do (or not to do) is to create a model that contains the feedback for one course and that is link to an exchange.
  • If you do so don't forget to modify your queryset to prefetch all the feedback (ask @chehabfl) for better performances.
  • If you do so, you can put the general feedback of the user on the exchange model (aka PreviousDeparture)
• The other possibility, is to stick to JSON, but I think the one above is better to start with.

You have to keep in mind that:

• The courses followed by a student will automatically created (and updated) given the data from the UTC database.

Some questions have to be resolved for this issue:

• What do you grade on each course (for now: adequation and quality; I don't recall why I chose "integration")
• #59 (closed)

Of course, don't forget about security:

• the info should be readable by everyone, but editable only by the user (and eventually a moderator (and above level)).
• For now, you can use a versioned model (that will automatically compress the successive version by the user, but will allow to keep track if an admin modify something).

Frontend

Well, then you need to build the front for this.

First display information, then add the possibility to edit. You might want to wait for #51 (closed) to be fixed first, but that's not necessary.