Allow Stacking Scripted Entities

[shadowscion]

Small modification to enable stackable scripted entities.

• the convar _force_allow_sents must be enabled on the server
• the entity's table must have .Stackable set to true

There is a slight problem regarding entity limits (not all scripted entities are meant to count against 'sents') but it shouldn't be a hindrance unless maxsents is set to something really low on the server.