Modifies ID computers to allow opening additional positions for latejoiners. Very restricted at the moment to test how it's used by the playerbase

See http://forums.nanotrasen.com/viewtopic.php?f=16&t=12490&p=195140#p195140 for details
Re-enabled closing original positions with a cooldown
Added logic checks to my stuff in topic() so you can't bork the server by griefing with byond://?src links
Forgot to remove a testing-output
Committing Elo001's job management stuff.

I made some very minor modifications (mainly tidied up the code a little bit so it's easier to read/modify).

I have made it so opened_positions can be negative. This means that reopening original positions, which were just closed, no longer requires an annoying cooldown period.

Removed the "AI" "Cyborg" "Assistant" checks since they were already handled within the blacklist.

The can_close_job and can_open_job procs now return codes for different reasons for failure (this removed a little bit of code duplication)

Nice work for a first commit, Elo :3
Merge branch 'master' of github.com:tgstation/-tg-station into Upstream