Add a command to abandon a grid

[drNovikov]

A player asked if there is a way to abandon a broken ship that ran away so he could have another "Unlicensed" in his fleet. Probably, a command, like the one on the Transcend servers.

Something like /gc fleet abandon CLASS NUMBER

That should remove the ship from player's fleet, disable all the blocks and switch the ownership to "nobody".

[zrisher]

Currently the problem with this is that someone can take over your ship and then be very surprised when you run this command and their "captured" ship starts to disappear.

After PR #114 is incorporated, we can base the ability to run this command off classifier ownership (like everything else), and this should be pretty safe.

This is actually already suggested in #14, but probably better to have a separate issue for it here now that most of #14 is done.

[zrisher]

I like /gc fleet disown N a littler better - more descriptive of the behavior