Try to merge the get_highest_level_cards() and get_next_level_cards()

[dallaszkorben]

In the
get_highest_level_cards() and
get_next_level_cards()
functions in the database.py there is only one difference in the database query:

in the get_highest_level_cards() it takes the highest level (or the specified level) from the recursive list
-------------------
-------------------
--- Conditional ---
--- filter ---
-------------------
-------------------

    --- if :level is set, then takes that specific level as highest level
    --- if :level is NOT set, then takes the highest level
    CASE
        WHEN :level IS NULL THEN id_higher_card IS NULL ELSE level = :level
    END

in the get_next_level_cards() it takes the one which has the given id as parent
-------------------
-------------------
--- Conditional ---
--- filter ---
-------------------
-------------------

    id_higher_card = :card_id