[CHORE] Remove references to "model", replace with "manifest" or "application"

[brooksmtownsend]

With the initial releases of wadm we had the notion of a "model", which has largely been replaced by "Application" in verbiage. Since developers don't ever interact with this idea of a "model", it can be confusing.

• When referring to the actual YAML or the manifest itself, we should call it a Manifest.
• When referring to the parsed + interpreted manifest, we should call it an Application

When in doubt, I think calling it an Application is good. I don't think we need to update the topics that wadm subscribes on quite yet, but any internal structures can be updated easily. Any public structs/enums/traits will need to be released with a breaking change, but that's no big deal.

[stale]

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