Devise links are not appended with non-ERB template engine

Since [Devise only generates ERB views](https://github.com/plataformatec/devise/issues/2455#issuecomment-18999729) (e.g. with `rails g devise:views`), running `rails g navigation` when the template engine is set to say, `slim` will result in nothing being appended to `_navigation_links.html.erb`.

I've relaxed the file check on Devise views to accept both `.erb` or whatever template engine is being used so that the links get generated as expected.