Removed redundant user authentication links

* "Sign in" and "Sign up" links no longer appear at the bottom of Devise-
  generated User-authentication-related views
* "Sign in" and "Sign up" links moved to partial that is rendered in application
  layout
* If in "Sign In" page, the "Sign In" link is not rendered; same for "Sign Up"
* Cleaned up links.html.erb
Merge pull request #4 from adwns42/change-nav-for-signup

Removed redundant user authentication links