Move banner form partial to a component

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

app/controllers/admin/banners_controller.rb

@@ -3,8 +3,6 @@ class Admin::BannersController < Admin::BaseController
 
   has_filters %w[all with_active with_inactive], only: :index
 
-  before_action :banner_sections, only: [:edit, :new, :create, :update]
-
   respond_to :html, :js
 
   load_and_authorize_resource
@@ -47,10 +45,6 @@ class Admin::BannersController < Admin::BaseController
        web_section_ids: []]
     end
 
-    def banner_sections
-      @banner_sections = WebSection.all
-    end
-
     def resource
       @banner ||= Banner.find(params[:id])
     end