generations: Create type for school year

[BrunoRosendo]

Currently, a generation is identified by a string named schoolYear that is validated by the SchoolYear validator. Its format is <xx-yy> where yy=xx+1.

This works fine but there are some places where we need to manage the years as integers and we might want to use school years for other models in the future. So, it'd be nice to create a SchoolYear type that makes it easier to get the school years and is always validated. We still want to save it as a string in the database, so a Converter might be useful, the same way that it's used for Permissions.

The current places where this would be useful are: SchoolYear validator inferSchoolYearIfNotSpecified in GenerationService. Please check if there are more cases when you implement this.