Move database logic into separate subdirectory

I'm trying to see if this speeds up builds a bit. May not without a
module boundary, but for now it's nice organization to have as the
program grows.

sql/user_by_username.sql

@@ -1,4 +0,0 @@
--- UserByUsername
-SELECT * FROM user_ WHERE
-	username = $1 AND
-	password_hash_type = 'bcrypt-14';