Fix managing widget cards for homepage

The condition `params[:page_id] != 0` didn't work properly when editing
the homepage because in that case the parameter was `nil`, and the line
`SiteCustomization::Page.find(@card.site_customization_page_id)` raised
an exception because it couldn't find a page with a `nil` ID.

Fixing the issue while maintaining the check against `0` lead to complex
code, and so allowing `nil` in the database and assuming cards with no
`site_customization_page_id` belonged in the homepage seemed to be the
easiest solution.

db/migrate/20181218164126_add_site_customization_page_to_widget_cards.rb

@@ -1,5 +1,5 @@
 class AddSiteCustomizationPageToWidgetCards < ActiveRecord::Migration
   def change
-    add_reference :widget_cards, :site_customization_page, index: true, default: 0
+    add_reference :widget_cards, :site_customization_page, index: true
   end
 end

db/schema.rb

@@ -1503,7 +1503,7 @@ ActiveRecord::Schema.define(version: 20181218164126) do
     t.boolean  "header",                     default: false
     t.datetime "created_at",                                 null: false
     t.datetime "updated_at",                                 null: false
-    t.integer  "site_customization_page_id", default: 0
+    t.integer  "site_customization_page_id"
   end
 
   add_index "widget_cards", ["site_customization_page_id"], name: "index_widget_cards_on_site_customization_page_id", using: :btree