Implement paging of article lists for AgentTicketZoom

Currently it is basically impossible to open a ticket with several thousand articles if there is any kind of server timeout, because the tickets will just render too slowly (and use up lots of RAM on the server).

This implements a basic pagination for the article box, with a configurable limit (default: 1000).  While this is certainly not perfect, care has been taken to change nothing (no measurable performance penalty, no layout changes) for tickets smaller than the configured limit.