Allow adding one login id only in api
Allow adding one login id only in api

When we integrate add login id api with interaction, we find that the
original add login id api will fire multiple create identity events which
is not desirable. After discussion, add multiple login id at the same
time is not typical, so change the api to allow adding one login id
only.

SkygearIO/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server,SkygearIO/skygear-server