Give task owners the ability to select their annotators

[TristanThrush]

Create a new section in the task owner interface that allows task owners to select annotators (similar to the section that allows task owners to add owners). If a selected annotator inputs an example, then insert into the metadata_json for that example that the example came from a selected annotator. Same with validations.

[TristanThrush]

@douwekiela is this ok, from a data privacy perspective? If the task owner selects only one annotator, then the task owner will be able to identify examples from this person. I think that this is unavoidable if we want to implement the feature that the hatemoji team needed.

If this feature isn't ok, then we could ask users to consent to being a "selected annotator" for a task before they are marked as one.