Store heroes played in matches table

[cheshire137]

This is like #71 but for heroes_matches instead of match_friends.

• Add hero_ids array field to matches table, populated from the heroes_matches table
• Update everything to work with heroes being on the match itself and not in a relation
• Add #prefill_heroes to avoid an n+1 query looking at the heroes in a match

[cheshire137]

SQL to run before:

SELECT match_id, ARRAY_AGG(hero_id ORDER BY hero_id) AS hero_ids FROM heroes_matches GROUP BY match_id ORDER BY match_id;

SQL to run after:

SELECT id AS match_id, hero_ids FROM matches WHERE hero_ids <> '{}' ORDER BY match_id;