Add trending links (#16917)

* Add trending links

* Add overriding specific links trendability

* Add link type to preview cards and only trend articles

Change trends review notifications from being sent every 5 minutes to being sent every 2 hours

Change threshold from 5 unique accounts to 15 unique accounts

* Fix tests

db/migrate/20211031031021_create_preview_card_providers.rb

@@ -0,0 +1,12 @@
+class CreatePreviewCardProviders < ActiveRecord::Migration[6.1]
+  def change
+    create_table :preview_card_providers do |t|
+      t.string :domain, null: false, default: '', index: { unique: true }
+      t.attachment :icon
+      t.boolean :trendable
+      t.datetime :reviewed_at
+      t.datetime :requested_review_at
+      t.timestamps
+    end
+  end
+end

db/migrate/20211112011713_add_language_to_preview_cards.rb

@@ -0,0 +1,7 @@
+class AddLanguageToPreviewCards < ActiveRecord::Migration[6.1]
+  def change
+    add_column :preview_cards, :language, :string
+    add_column :preview_cards, :max_score, :float
+    add_column :preview_cards, :max_score_at, :datetime
+  end
+end

db/migrate/20211115032527_add_trendable_to_preview_cards.rb

@@ -0,0 +1,5 @@
+class AddTrendableToPreviewCards < ActiveRecord::Migration[6.1]
+  def change
+    add_column :preview_cards, :trendable, :boolean
+  end
+end

db/migrate/20211123212714_add_link_type_to_preview_cards.rb

@@ -0,0 +1,5 @@
+class AddLinkTypeToPreviewCards < ActiveRecord::Migration[6.1]
+  def change
+    add_column :preview_cards, :link_type, :int
+  end
+end