Introduce api for changing the identity of an existing user
Description
Currently I've been messing around with automatically creating users in gitlab-ce, adding existing ssh-keys to them and afterwards, changing their identity to a different provider, in my case atlassian crowd. I tried to set it when doing a post request on user creation by adding `{ 'identities': [ { 'provider': 'crowd', 'external_uid': 'foobar' } ] }``into my request, but it gets ignored.
Proposal
There should be a possibility to set the identity of a user programmatically using rest.