Editing changes to the Projects Resource

* Use the correct example resource for the nested import in a project
* Include notes about the custom error codes when
  deleting or merging projects
* Fix the example responses to be wrapped in JSON arrays
* Only allow a specific set of accepted values for the billing
  increment
* rename associated resource field names to `*_id(s)` where
  appropriate
* Minor copy changes.
Update custom error codes in Expense update/delete, mark as finished

* Expenses cannot be updated or deleted if they have been invoiced or
  are in an archived project.
* The resource is also ready to be used.