Special Repetition rewards

[BONNe]

Is your feature request related to a problem? Please describe.
This feature request comes from #20.
Special repetition rewards which users could get by completing a challenge, for example, every 10th time.

Describe the solution you'd like
Add new reward option: Special, that can be triggered by custom options, f.e., after some repetition count, each X repetition time or at the last time (if it is set).

[tastybento]

I've reviewed this against the current code. Challenges support two reward tiers today — first-time rewards and repeat rewards (repeatMoneyReward, repeatExperienceReward, repeatItemReward) — but there's no way to give something special on the Nth repeat.

A simple milestone model is doable without complicating the common case:

1. A list of milestone entries on Challenge (repetition count → extra rewards/commands), empty by default.
2. TryToComplete's reward handling checks the player's completion count and applies any matching milestone on top of the normal repeat reward.
3. Admin GUI support for editing milestones.
4. JUnit tests to verify milestones fire at exactly the right counts.

Keeping this open as P2 — worth having, but it touches the core completion path and GUI, so it needs to be done carefully.