Major custom folder update!
These scripts have been chosen for being unique and useful, yet structurally simple. In this way, they can serve as usable reference material for future user scripts.

Contents:
* Battleground: bg_emp (Emperium breaker), bg_pvp (PVP match)
* Event: mvp_ladder (kill every MVP to win), devil_square (3-round monster summoning event)
* Utility: card_seller (sells all monster cards), mvp_room (private MVP summoning room), autopot (automatically use potions when hit)
Credits to AnnieRuru for offering up her scripts and modifying them for use.

Other changes:
* Moved previous custom battleground scripts into 'battleground/unofficial/'.
* Moved custom holiday event scripts into 'event/holiday/'.

Signed-off-by: Euphy <euphy.raliel@rathena.org>