move dashboard to its own route

this splits the concept of 'home' so that `loomio.org` will always show the frontpage.

when you're logged in, if you want to see your dashboard that is now at   `loomio.org/dashboard`

```ruby
root_path should == '/'
dashboard_path should == '/dashboard'
```

there are a number of places in the app where completion of actions should return you to your dashboard. these are things like saving settings, or being rejected from trying to access a group you're trying to access, and clicking the loomio logo in the nav-bar