"Who Am I?" – log in with Personas

Pivotal Tracker ticket [#75075362](https://www.pivotaltracker.com/story/show/75075362), "As a City stakeholder, I want users of this system to have to log in before they can see any sensitive data."

Known TODOs:
- [ ] write tests
- [ ] resolve merge conflicts with master
- [x] prevent users from accessing pages besides '/' if they're not logged in
This doesn't work as-is (should be `if current_user.is_authenticated():`), but maybe we can import and use Flask-Login's login_required? https://flask-login.readthedocs.org/en/latest/#flask.ext.login.login_required

It looks like the best way to do that would be to remove this, import login_required, and define an `unauthorized` callback. (It looks like you can use the `unauthorized_handler` decorator on a function to write that callback).