API resources now support a couple new query arguments for limiting the

results in a payload, in order to reduce database queries and payload

sizes.
The ?only-fields= query argument limits the returned fields in the

payload to the comma-separated list of field names. If the value is

blank, then no fields will be returned, leaving only links.
Likewise, the ?only-links= query argument limits the returned links in the

payload. It behaves exactly like ?only-fields=.
Resources that inject custom fields into the payload outside of

serialize_object() can call get_only_fields() and get_only_links() to

determine whether to include specific fields.

Unit tests pass.
Tested this pretty extensively with changes to rbt post and with custom

curl commands on several resources in Review Board.