private messaging does not work with external sbot

[cryptix]

If you try to use the private messaging with an external sbot (-g flag) you will just get a blank page and the following error in the inspector:

TypeError: this._sbot.private.read is not a function"

This is because vanilla scuttlebot does not use the ssb-private module which offers the indexing read method. Once ssbc/ssb-server#417 is merged, this should just work.