"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:
- [x] make login page
- [x] write tests
- [x] resolve merge conflicts with master
- [x] 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