Grab school year from the competition instead of date

[shippy]

Each school year in question should include both the previous grand finals debate and the final tournament. Currently, however, the school_year is determined by

debates_in_years as (
    select
        *,
        date_sub('year', DATE '{{ var("first_school_year") }}', datum) as school_year
    from debates
),

in base__debata.sql. Instead, we should look at via source("raw", "debata").soutez_id into source("raw", "soutez").rocnik and use that. (It should be a match for the school_year stat, but we should double-check that.)

Therefore, to implement, we must:

1. Add a CTE that extracts soutez_id, rocnik from source("raw", "soutez"), which should also be added to base/sources.yml`.
2. Move current school_year column to school_year_calc and keep it in the debates_in_years CTE
3. Rename the rocnik column from the first added CTE to school_year in debates_in_years CTE
4. Double-check that max(school_year) == var("current_school_year")