"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
- [ ] 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
@dget Following up on our convo the other day: we shouldn't need to worry about the use case of coming back to an old tab after an hour or so, as our current policy requires us to implement automatic logout after 15 minutes (of inactivity, let's hope, not period).