Feature/193 student acceptance

[auroranavas]

This PR aims to change the status of a Student when a year since status = ACEPTADO has passed.
First, I tried to implement it so it was done automatically, ie asynchronously. However, since Django is a synchronous framework it, would have to be done with third-party libraries. By researching this, I saw that this approach was more complicated than it seemed and didn't seem worth it.

So, what I did was introduce a field in Student called acceptance_date. I have overridden the retrieve method do that every time a GET is made, it checks if it has been more than a year since acceptance_date. If more than a year has passed, the status changes to RECHAZADO.

Note for Frontend developers: When a Student is created, the acceptance_date should be left null (don't send anything to backend). When the admin accepts the Student, then you should send the acceptance_date as Date.today.

[JuanluRM]

The retrieve method seem to work as intended but the issue is that if we don't call students thought the view, it doesn't get updated. This as far as I know only happens through the export function, which means if an admin wants to export students it won't update the status so for example it will appear "ACEPTADO" when should be "CADUCADO" because a year has passed. I ask you to fix that so it checks before exports as well.

[auroranavas]

I think that what you requested should be covered now. Thank you for your feedback 😊.
By the way, most of the lines that appear edited by me is formatting made by Black in my IDE.

[JuanluRM]

Everything seems to be working well now, remember to forward the note for front end so they update the acceptance_date every time a student is accepted. Well done.