Clarify Job vs Attachment states and use enumertations throughout

There are a variety of different statuses tracked in Testflinger and the accompanying GitHub actions. Most of these should to be tracked with enumerations rather than raw strings.
- job status: enum exists and should be used throughout
- attachment status: enum required and ought to be used throughout
- GitHub action status ("complete") not sure if we can use an enum in the GH actions, but be careful with search and replace

Within the **JobState** enum, presently there are two confusing states:
- ALLOCATE = "allocate"
- ALLOCATED = "allocated": the state after "allocate" should have a different name