Move banner form partial to a component

Other than simplifying the controller, this'll make it easier to write
tests for this code.

app/components/admin/banners/form_component.rb

@@ -0,0 +1,15 @@
+class Admin::Banners::FormComponent < ApplicationComponent
+  include TranslatableFormHelper
+  include GlobalizeHelper
+  attr_reader :banner
+
+  def initialize(banner)
+    @banner = banner
+  end
+
+  private
+
+    def sections
+      WebSection.all
+    end
+end