include links in GET responses for builds and racks
allow updating links in
POST /build/:build_id_or_name
POST /rack/:rack_id_or_name
new endpoints:
POST /build/:build_id_or_name/links
DELETE /build/:build_id_or_name/links
POST /rack/:rack_id_or_name/links
DELETE /rack/:rack_id_or_name/links
include links in GET responses for builds and racks
allow updating links in POST /build/:build_id_or_name POST /rack/:rack_id_or_name
new endpoints: POST /build/:build_id_or_name/links DELETE /build/:build_id_or_name/links POST /rack/:rack_id_or_name/links DELETE /rack/:rack_id_or_name/links
closes #928, #1040.