Add basic http auth for admin

[samselikoff]

When user first sets up feb with a backend, if there's no app they need some way to authenticate to access the admin

front_end_admin http_auth: true

[hhff]

:+1: often have the rails app setup just to host the ember app for staging purposes. def need this!

[anilmaurya]

Until this feature is added, use this

protected_app = Rack::Auth::Basic.new(FrontEndBuilds::Engine) do |username, password|
   username == ENV['username'] && password == ENV['password']
end

mount protected_app, at: '/admin'