Rework server inventory data model

[celskeggs]

We need a couple of things to change about how we handle server inventory data in hwops.mit.edu:

• We need a history of what the state of the machine has previously been, so that we can see how it has changed over time.
• We need a distinction between atemporal notes (like "the interface is at X.Y.Z") and temporal comments (like "I took an action about this")

This means reworking the data model and the interface.

[celskeggs]

Implemented the main part in ba85529. I haven't yet implemented the comments/notes thing, but I did rename 'comments' to 'notes'.