Redirect to sign-in page for any action that requires authentication

Pivotal tracker issue:

https://www.pivotaltracker.com/story/show/55865366

This makes Devise redirect to login page whenever an action requires authentication. The vast majority of these use (a variant) of this single line in the controller:

```ruby
before_filter :authenticate_member!, :except => [:index, :show].
```

I've added a test to check if it redirects to the login page to read notifications, as that was the problem reported. I don't know if it's worth adding tests for the rest of them, as that feels like we'd be testing Devise's functionality...