2 Slim Issue fixes with foundation5

(sorry, there's two issues here - kinda new to submitting patches and wasn't sure how to separate the pull requests)

1. Fix _navigation_links.html.erb copy so that it uses the appropriate template engine
Generating the foundation5 layout with slim (w/ the `slim-rails` gem so that the templating engine gets set properly) caused `app/views/layouts/_navigation_links.html.erb` to be created - all other files were good.  This one should be `.slim` instead.  Looks like this will also fix other non-erb engines.

2. Fix the close button nesting for the foundation5 slim template.  As it stands, it appears below the alert box instead of inside it. 

 **I noticed that other slim `#{framework}-messages.html.slim` template has similar nesting issues that I was tempted to also fix and submit, but I don't know much about those frameworks myself**