perf(game): add composite indexes for game_id and achievements unlocked

[wescopeland]

This dramatically improves performance on the two queries to fetch the game's achievement distribution on game pages.

Benchmark query:

-- softcore mode, use NOW() to bypass local performance caching
SELECT
	NOW(),
	pg.achievements_unlocked AS AwardedCount,
	COUNT(*) AS NumUniquePlayers
FROM
	player_games AS pg
WHERE
	pg.game_id = 228
	AND pg.achievements_unlocked > 0
GROUP BY
	AwardedCount
ORDER BY
	AwardedCount DESC

Before
4.389s
4.697s
4.282s
4.723s
4.867s
Average: 4.591s

After
38ms
23ms
8ms
24ms
19ms
Average: 22.4ms

The addition of these indices results in a speedup factor of around 535x. This is especially desirable given the query runs twice on every game page.

[Jamiras]

I'm not sure if the NOW() is enough to bypass my local caching.

Before: 69ms, 71ms, 67ms, 70ms, 68ms (average: 69ms)
After: 8ms, 9ms, 8ms, 8ms, 8ms (average: 8ms)

Still a 8x improvement.

For 1446:

Before: 149ms, 78ms, 116ms, 94ms, 230ms (average: 133ms)
After: 5ms, 5ms, 8ms, 7ms, 6ms (average: 6ms)

That's a 22x improvement

[luchaos]

runs twice on every game page

Should probably be pre-aggregated on games/sets. Won't scale well in the future.
The index does its job though, very nice!