Add sitepress in "Rails" mode

[benjaminoakes]

https://sitepress.cc/

• Add gem "sitepress-rails", "~> 4.0"
• Run bin/rails generate sitepress:install
• Confirm sitepress_pages and sitepress_root in routes
• Remove commented out root route
• Work around Slim pages are not rendering / not found sitepress/sitepress#58 by adding config/initializers/sitepress.rb

# https://github.com/sitepress/sitepress/issues/58
ActionView::Template::Handlers.extensions << :slim