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/packages/security.yaml

@@ -27,6 +27,8 @@ security:
             remember_me:
                 secret: '%kernel.secret%'
                 lifetime: 604800 # 1 week in seconds
+            login_throttling:
+                max_attempts: 5
 
     access_control:
         - { path: ^/admin, roles: ROLE_ADMIN }
@@ -43,3 +45,7 @@ when@test:
                 cost: 4 # Lowest possible value for bcrypt
                 time_cost: 3 # Lowest possible value for argon
                 memory_cost: 10 # Lowest possible value for argon
+        firewalls:
+            main:
+                login_throttling:
+                    max_attempts: 2