Fixed importScopedSession() and moved exportUserSession() to RequestContext.

* Renamed WebRequest::exportUserSession -> RequestContext::exportSession.
  Updated the only callers of this new function.
* Init the user with User::newFromId() instead of relying on the session
  (which breaks when things like CentralAuth are enabled).
* Made RequestContext::exportSession() include the user ID.
* Removed now-redundant user ID checks in upload jobs.
* Added unit tests for the session import function.

Change-Id: I543e6766f7a8a828ea5d270328c3bc7738c6fe94
(cherry picked from commit f31a0463aaee6f8cc18d05c3af4804f67f36a2c1)