pass through assign attrs in link#navigate helper

[jprincipe]

I noticed that the keyboard navigation of the Sidebar User Menu wasn't working for anything except the Sign Out link. The issue was the attributes that are passed in to the other menu items (i.e. role="menuitem") were being dropped in the live link helper:

def link(%{navigate: _to} = assigns do
   # ...
end

Rather than passing along all incoming assigns as attributes, it was extracting just the class assign and using that. This PR just updates that link/1 function so it works like the other link/1 helpers.

I'd also like to thank you for putting together a project like this. Invaluable resource for learning the new features of LiveView.