Adds nullable creator field, containing a reference to the user who
created the stream. When creating a stream, acting user is set as
the creator of the stream. Since API calls to create streams always
have an acting user, this field should always be set when streams
are created using the API.
Because streams can be created with no acting user, this field is
nullable. We try to backfill existing streams using RealmAuditLog table,
but not all streams are guaranteed to have a recorded create log. Thus
this new field is left null when it cannot be backfilled. We also set
this field to null when the creator user is deleted.
tnmkr