user auth

## User Auth Rails app: What we're adding:
- User model
```bash
rails g model user username:string password_hash:string
```
- user_id foreign key in tables (user has many cars)

generate a migration
```bash
rails generate migration addUserId
```

add the foreign key to the cars table
```ruby
class AddUserId < ActiveRecord::Migration
  def change
    add_reference :cars, :user
  end
end
```
- a login form

this requires some routes:
```ruby
  get    'login'   => 'sessions#new'
  post   'login'   => 'sessions#create'
  delete 'logout'  => 'sessions#destroy'
```

```ruby
<%= form_for(:session, url: login_path) do |f| %>

    <%= f.label :username %>
    <%= f.text_field :username %>

    <%= f.label :password %>
    <%= f.password_field :password %>

  <%= f.submit "log in"%>
<% end %>
```
- a register form
```ruby
  post   'login'   => 'sessions#create'
```

```ruby
<%= form_for(:user, url: login_path) do |f| %>

    <%= f.label :username %>
    <%= f.text_field :username %>

    <%= f.label :password %>
    <%= f.password_field :password %>

    <%= f.label :password_confirmation %>
    <%= f.password_field :password_confirmation %>

  <%= f.submit "log in"%>
<% end %>
```

- an edit user form