…elastic#43232)

* Refactor watch serialization logic into common serialization functions.
  - Refactor build helpers to accept specific arguments instead of the entire watch object, to make dependencies more obvious.
  - Move action models into common directory.
* Remove boom error reporting from action models because this is an unnecessary level of defensiveness since we control the UI that consumes this API.
* Convert tests from Mocha to Jest.
  - Remove mocks and fix assertions that depended upon mocked dependencies. These assertions were low-value because they tested implementation details.
  - Remove other assertions based upon implementation details.
* Remove serializeMonitoringWatch logic, since Monitoring doesn't use the create endpoint.