Core/Commands: implement .pvpstats command

Shows number of battleground victories in the last 7 days

Closes #13460

Conflicts:
	sql/base/auth_database.sql
	sql/updates/auth/2014_11_10_00_auth.sql
	src/server/game/Accounts/RBAC.h

(cherry picked from commit fd0b96178174e624510a10f5bc27b60af406d901)