The e-mail forwarder will use this. Set it to nullable temporarily to
accomodate existing streams; later commits will a) provide a script to
give all streams a token, and b) make the field non-null.
Realm administrators will eventually have a UI to regenerate stream
tokens.
(imported from commit a084d0a7012eb9665e4da095cbc46aa9ef354eaa)
c020545e02