Add conversation model, <ostatus:conversation /> (#3016)

* Add <ostatus:conversation /> tag to Atom input/output

Only uses ref attribute (not href) because href would be
the alternate link that's always included also.

Creates new conversation for every non-reply status. Carries
over conversation for every reply. Keeps remote URIs verbatim,
generates local URIs on the fly like the rest of them.

* Fix conversation migration

* More spec coverage for status before_create

* Prevent n+1 query when generating Atom with the new conversations

* Improve code style

* Remove redundant local variable

db/migrate/20170506235850_create_conversations.rb

@@ -0,0 +1,10 @@
+class CreateConversations < ActiveRecord::Migration[5.0]
+  def change
+    create_table :conversations, id: :bigserial do |t|
+      t.string :uri, null: true, default: nil
+      t.timestamps
+    end
+
+    add_index :conversations, :uri, unique: true
+  end
+end

db/migrate/20170507000211_add_conversation_id_to_statuses.rb

@@ -0,0 +1,6 @@
+class AddConversationIdToStatuses < ActiveRecord::Migration[5.0]
+  def change
+    add_column :statuses, :conversation_id, :bigint, null: true, default: nil
+    add_index :statuses, :conversation_id
+  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: 20170507141759) do
+ActiveRecord::Schema.define(version: 20170508230434) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -62,6 +62,19 @@ ActiveRecord::Schema.define(version: 20170507141759) do
     t.index ["account_id", "target_account_id"], name: "index_blocks_on_account_id_and_target_account_id", unique: true, using: :btree
   end
 
+  create_table "conversation_mutes", force: :cascade do |t|
+    t.integer "account_id",      null: false
+    t.bigint  "conversation_id", null: false
+    t.index ["account_id", "conversation_id"], name: "index_conversation_mutes_on_account_id_and_conversation_id", unique: true, using: :btree
+  end
+
+  create_table "conversations", id: :bigserial, force: :cascade do |t|
+    t.string   "uri"
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["uri"], name: "index_conversations_on_uri", unique: true, using: :btree
+  end
+
   create_table "domain_blocks", force: :cascade do |t|
     t.string   "domain",       default: "", null: false
     t.datetime "created_at",                null: false
@@ -255,7 +268,9 @@ ActiveRecord::Schema.define(version: 20170507141759) do
     t.integer  "favourites_count",       default: 0,     null: false
     t.integer  "reblogs_count",          default: 0,     null: false
     t.string   "language",               default: "en",  null: false
+    t.bigint   "conversation_id"
     t.index ["account_id"], name: "index_statuses_on_account_id", using: :btree
+    t.index ["conversation_id"], name: "index_statuses_on_conversation_id", using: :btree
     t.index ["in_reply_to_id"], name: "index_statuses_on_in_reply_to_id", using: :btree
     t.index ["reblog_of_id"], name: "index_statuses_on_reblog_of_id", using: :btree
     t.index ["uri"], name: "index_statuses_on_uri", unique: true, using: :btree