Rebirth fixes: deequip pets and mounts, allow stacking at max level

Two tweaks to Rebirth behavior:

* Undergoing a Rebirth deequips the user's current pet and mount, if any.
* In the unlikely, hardest-core event that a player reaches level 100 more than once, they can stack Rebirths there as many times as they make it. (Previously they needed to exceed their prior level to re-earn the achievement, making it impossible to stack the Rebirth achievement after being reborn from lvl 100.)