Order bot should consider requisition state and ping different groups

[KTOmega]

Right now, the Order Bot just pings the requisition requestor whenever a change is made. It would be helpful to ping more people -- for example, pinging the approvers once a requisition is placed or pinging a purchaser once a requisition is approved.

This would require implementing business logic to inspect the state of the requisition. Currently, the bot does not do any logic based on requisition state (besides deciding what to display/output to Discord).