Improvements to courier upgrading and usage

[Riveon]

• Supports seem to upgrade courier at the 5:30 minute mark or later, not at 3:00 (judging by your code it seems like they should already be upgrading it at 3 minutes).
• Eventually, certain roles (e.g. mid, carry) should get a higher priority for courier usage, particularly in the early game.
• Eventually, it would benefit bots (especially cores and carries) a lot if they bought salves/clarities when low on either mana or health and called them over via courier (if shrine not available).
  These last two suggestions aren't important right now though I realise.