Fix selser in the v2 API
- It was setting env.page.dom as the document, whereas throughout the codebase that's usually the body. This produced an unnecessary diff.
- To this point, some of the tests for the v2 API are merely documenting the expectations, and not enforcing anything. Here we add a test to ensure selser is used.
- Turns on useSelser in the API test parsoidServer.
- Renames some variables in the selectiveSerializer to reflect that we're working with a body, not a doc.
- Clarified that a new data-parsoid should not be submitted. Can only be found in the original field and applied to both new and old html.