ShadowRift Portal timer to close

Added from one of Zaphod's wish lists - this is to stop portals being opened and left open for an infinite time with a botting mule.
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Added a timer to the constructor to close the portal after a given time.

Made a timer tied to viMultiCast_SpellPower (which is used to determine
how long it takes to cast the spell) which is also editable in the $
screen in game. My thought process behind this was, making the spell
harder/longer to cast should make the portal last longer....

Necro Portals will now last a random time between the lowest timer of 20
mins and the highest timer of 30 mins after timer elapses the portal
closes itself.

viMultiCast_SpellPower is currently 1500
1500 * 800 = 20 mins (lowest roll)
1500 * 1200 = 30 mins (highest roll)

I think 20-30 mins is fair.