Add indexes to downloads table to make selects faster

The combination of a WHERE, ORDER BY, and LIMIT meant that we were
ordering entries first before filtering the relevant ones when showing
lists of sounds, packs, and people. Instead, we create three new
composite indexes:

sounds_download(sound_id, created):
For people who downloaded a specific sound

sounds_download(user_id, created)
For sounds downloaded by a specific user

sounds_download(pack_id, created) WHERE pack_id IS NOT NULL
For packs downloaded by a specific user
We have a partial index where pack_id is not null because there are
so many sounds in this table - we don't want to have to always
skip over these rows
Merge branch 'speedups'