When using Gerrit with GitHub authentication, we need to "remember"
the user's federated auth token in order to allow Gerrit to impersonate
the user when communicating with GitHub.
The user is always in control as the external identity is listed in the
user's external ids and can be removed / revoked at any time.
This change is *mandatory* for being able to integrate the GitHub group
backend system properly, as GitHub does not allow anonymous browsing of
organisations and groups (very hard 60 calls/hour cap) and Gerrit needs
to impersonate the user to understand to which organisation / group he
belongs to.
NOTE: each user can have at most 1 external identity as at the moment is
not possibly to authenticate against multiple external systems at once.
Change-Id: I8df2540a5643c95d51b26e9d3e32f6cd1cac9f10
97d073567c