Add rate limiting to login and season-code entry points

Neither /login nor the public season-code guess form (POST /) had
any throttling, making both an unlimited brute-force/enumeration
oracle. Adds symfony/rate-limiter and enables login_throttling on
the main firewall, plus a dedicated per-IP rate limiter on the
season-code form.

config/reference.php

@@ -629,7 +629,7 @@ use Symfony\Component\Config\Loader\ParamConfigurator as Param;
  *         }>,
  *     },
  *     rate_limiter?: bool|array{ // Rate limiter configuration
- *         enabled?: bool|Param, // Default: false
+ *         enabled?: bool|Param, // Default: true
  *         limiters?: array<string, array{ // Default: []
  *             lock_factory?: scalar|Param|null, // The service ID of the lock factory used by this limiter (or null to disable locking). // Default: "auto"
  *             cache_pool?: scalar|Param|null, // The cache pool to use for storing the current limiter state. // Default: "cache.rate_limiter"