Extract signed in and signed out nav to seperate partials

Admittedly this is a bit overkill for our existing situation; but this
pattern is very powerful. When you encounter swathes of template inside
of if/elsif/else logic, extract a method to tell you which partial to
use and pull each block into a partial.
Test and migration for adding a nickname field to user

I want users to be able to choose a public nickname. Unfortunately, this
means I have to get beyond the built in devise views.
`rails g devise:views`

So many forms, so little time!
Add nickname to registration form

Rails has the notion of "strong parameters" which help protect your
models from exploitation by an unsavory character. This means we have
to tell devise that we *want* peopl to be able to provide their nick
name on signup. We do this in our ApplicationController using a before
filter restricted to devise_controllers

We also add the users nickname to the welcome screen. This allows a user
to visually confirm that they are logged in as the correct person and
inspire them to logout when they leave.