Change revisions to record userid rather than username

[rgrp]

The use of username is problematic because username's can change.

• Change all revision creation code to use user id (simplest is to change c.author field in lib/base.py (?))
• Change user view page to look up against user id rather than name
• Perform migration on existing Revision objects
  • Match should probably be against both openid and username when searching Revisions' author field (especially true on CKAN where some people have already changed their username from being their openid)