Make PreviewCard records reuseable between statuses (#4642)

* Make PreviewCard records reuseable between statuses

**Warning!** Migration truncates preview_cards tablec

* Allow a wider thumbnail for link preview, display it in horizontal layout (#4648)

* Delete preview cards files before truncating

* Rename old table instead of truncating it

* Add mastodon:maintenance:remove_deprecated_preview_cards

* Ignore deprecated_preview_cards in schema definition

* Fix null behaviour

db/migrate/20170901142658_create_join_table_preview_cards_statuses.rb

@@ -0,0 +1,7 @@
+class CreateJoinTablePreviewCardsStatuses < ActiveRecord::Migration[5.1]
+  def change
+    create_join_table :preview_cards, :statuses do |t|
+      t.index [:status_id, :preview_card_id]
+    end
+  end
+end