Add tombstones for remote statuses (#9830)

* Add Tombstone model to remember object deletion

* Do not recreate a status if it has been deleted

* Record Tombstone for remote deleted items

Also, only record deleted items from same-host actors

* Clear an user's tombstones when their key change

db/migrate/20190117114553_create_tombstones.rb

@@ -0,0 +1,12 @@
+class CreateTombstones < ActiveRecord::Migration[5.2]
+  def change
+    create_table :tombstones do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }
+      t.string :uri, null: false
+
+      t.timestamps
+    end
+
+    add_index :tombstones, :uri
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_01_03_124754) do
+ActiveRecord::Schema.define(version: 2019_01_17_114553) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -615,6 +615,15 @@ ActiveRecord::Schema.define(version: 2019_01_03_124754) do
     t.index ["name"], name: "index_tags_on_name", unique: true
   end
 
+  create_table "tombstones", force: :cascade do |t|
+    t.bigint "account_id"
+    t.string "uri", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["account_id"], name: "index_tombstones_on_account_id"
+    t.index ["uri"], name: "index_tombstones_on_uri"
+  end
+
   create_table "users", force: :cascade do |t|
     t.string "email", default: "", null: false
     t.datetime "created_at", null: false
@@ -743,6 +752,7 @@ ActiveRecord::Schema.define(version: 2019_01_03_124754) do
   add_foreign_key "statuses_tags", "tags", name: "fk_3081861e21", on_delete: :cascade
   add_foreign_key "stream_entries", "accounts", name: "fk_5659b17554", on_delete: :cascade
   add_foreign_key "subscriptions", "accounts", name: "fk_9847d1cbb5", on_delete: :cascade
+  add_foreign_key "tombstones", "accounts", on_delete: :cascade
   add_foreign_key "users", "accounts", name: "fk_50500f500d", on_delete: :cascade
   add_foreign_key "users", "invites", on_delete: :nullify
   add_foreign_key "users", "oauth_applications", column: "created_by_application_id", on_delete: :nullify