Simplify widget table prefix

The same way it's done in other modules.

app/models/widget.rb

@@ -0,0 +1,5 @@
+module Widget
+  def self.table_name_prefix
+    "widget_"
+  end
+end

app/models/widget/card.rb

@@ -5,9 +5,6 @@ class Widget::Card < ApplicationRecord
     foreign_key: "site_customization_page_id",
     inverse_of:  :cards
 
-  # table_name must be set before calls to 'translates'
-  self.table_name = "widget_cards"
-
   translates :label,       touch: true
   translates :title,       touch: true
   translates :description, touch: true

app/models/widget/feed.rb

@@ -1,6 +1,4 @@
 class Widget::Feed < ApplicationRecord
-  self.table_name = "widget_feeds"
-
   KINDS = %w[proposals debates processes].freeze
 
   def active?