Add indexes to sha256_hash on media/thumbnail tables

CAUTION: This might take a while

Fixes https://github.com/turt2live/matrix-media-repo/issues/133

migrations/8_sha256_indexes_down.sql

+DROP INDEX IF EXISTS idx_sha256_hash_media;
+DROP INDEX IF EXISTS idx_sha256_hash_thumbnails;

migrations/8_sha256_indexes_up.sql

+CREATE INDEX IF NOT EXISTS idx_sha256_hash_media ON media (sha256_hash);
+CREATE INDEX IF NOT EXISTS idx_sha256_hash_thumbnails ON thumbnails (sha256_hash);