Refactored question service and added new category: 'Guess the city'

[ChristianFN2]

Now to create a new category we only have to formulate the sparql query in the format I wrote in the documentation of the method and the question will be generated accordingly, no need to create a new method for these type of questions, just create a new endpoint and use the parametrized method with the adjusted query.

Also, the answer endpoint is made to be universal for all categories of the same type, so there is no need to create a new one for each category, only if a new type is created.

[ChristianFN2]

I have to note also that, in the case of the city images, they could be resized better as they have quite variable sizes in contrast with flag images.

[adriiglz]

Refactored code looks great for me, and the new cathegory proves it. We can now extend the number of cathegories easily