Refactor slot and professor spiders due to changes to sigarra HTML

[tomaspalma]

The HTML page now looks like the following:

In the spider code we assume an hardcoded structure of the HTML as well as the names for the classes, which have changed, thus making the scrapper work worse.

There is an API which the javascript code from the sigarra schedule page uses and that the scrapper might want to use.
The link to said API does not need to be computed by us as it is inside the properties of one of the elements present in the HTML code of the page.

This is huge, since instead of havnig a complicated logic in order to retrieve the items, we just find the html element that has the link to the API and then make a request to it, which will hopefully make the code more understandable.