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"
http://apidock.com/rails/ActionView/Helpers/UrlHelper/link_to_unless_current