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