"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
- [ ] add login_required to the browse page
- [x] remove references to session['email']
- [x] prevent users from accessing pages besides '/' if they're not logged in
@dget Correction to the above: the example was from http://flask.pocoo.org/docs/patterns/viewdecorators/. So, yep, it was a reimplementation. Got the decorator working properly in 6bf81d9.