New Puzzle Theme: Kill Box Mate

[Siderite]

Since it is one of the practice checkmates patterns (https://lichess.org/practice/checkmates/checkmate-patterns-iv/96Lij7wH/tLcHpjXr) maybe it should be added as a puzzle theme, too.

There is a user that already made the effort to find all puzzles with this pattern (https://lichess.org/study/search?q=owner%3Adirkster98+Kill+Box+Mate and https://lichess.org/study/search?q=owner%3Adirkster99+Kill+Box+Mate)

Even if the puzzle theme is just added, we can work on voting it up for the correct puzzles.

[ornicar]

[ornicar]

And I gave that user custom permissions that make their puzzle votes and themes weight more.