Rename uuid property to id in REST API

[matthew-white]

We use different names for the entity UUID in different places:

• When using an entity in a form, the UUID is the name of the secondary instance.
• When creating or updating an entity in a form, the UUID is the id attribute of the <entity> element.
• In the integrity URL, we're planning to use id for the query parameter and the node name: Add an element in the mediaFile block for Entity Lists in the OpenRosa manifest to specify a URL to get Entity information #668
• In the REST API, the UUID is called uuid.
• In OData, it's __id.

The goal of this issue is to enhance consistency by replacing uuid in the REST API with id. Maybe we could do so in a way that would be backwards compatible (e.g., returning both id and uuid).

That said, there is some precedent within the REST API of naming the unique identifier something other than id. Submissions are uniquely identified by their instance ID, but the instance ID is named instanceId within the REST API, not id.

[matthew-white]

We discussed this on a call yesterday. Renaming uuid to id in the REST API would be a breaking change, so we've decided not to make this change. Going forward for new features in the future, we'll try to set up more consistency between names in Collect vs. Central.