Our file attachments can be in many different states, but we don't really have
a solid concept of these states in our codebase so far. We also don't have a
formal way of determining which state a file attachment is in.
This change introduces file attachment states and allows us to efficiently
get the state of a file attachment through its review request or draft.
This sets the foundation for an upcoming change that improves how we display
file attachments on a review request and how users can manage them.
