problems with tasks, task schemas

The v2/schemas/tasks and v2/schemas/task responses aren't incorrect, but they're not as accurate as they could be.

task schema:
- missing "links": [{"href":"{schema}","rel":"describedby"},{"href":"{self}","rel":"self"}]
- should have "additionalProperties": false
- some properties are already marked "readOnly":true , but there are other properties (e.g., "id") that should also be marked as read only
- could have a list of required properties added (I think that would be all except expires_at; the schema says it can have a null value, but i think in practice it doesn't appear in the response unless it's non-null)

tasks schema:
- same as the above readOnly situation
- same as the above required and expires_at situation