Simplify rendering a banner

Now the banner component accepts either a banner or a section and loads
the banner if it's a section, so we don't have to add the `@banners`
variable in several controllers.

app/controllers/welcome_controller.rb

@@ -11,7 +11,6 @@ class WelcomeController < ApplicationController
     @header = Widget::Card.header.first
     @feeds = Widget::Feed.active
     @cards = Widget::Card.body
-    @banners = Banner.in_section("homepage").with_active
     @remote_translations = detect_remote_translations(@feeds,
                                                       @recommended_debates,
                                                       @recommended_proposals)