Add user model and populate it after login

- Uses id from wiki as user id, reduces pain
- Puts user object in flask.g for every request.
  Does a database read now, should probably be cached
  at some point.
- Build base Model class and Session with SQLAlchemy
- Add 'createdb.py' to create database from scratch

Change-Id: I632044d881fa82047517ca09539c0a81bcb35207