Store URIs of follows, follow requests and blocks for ActivityPub (#7160)

Same URI passed between follow request and follow, since they are
the same thing in ActivityPub. Local URIs are generated during
creation using UUIDs and are passed to serializers.

app/models/follow.rb

@@ -9,6 +9,7 @@
 #  account_id        :bigint(8)        not null
 #  target_account_id :bigint(8)        not null
 #  show_reblogs      :boolean          default(TRUE), not null
+#  uri               :string
 #
 
 class Follow < ApplicationRecord
@@ -26,4 +27,16 @@ class Follow < ApplicationRecord
   validates :account_id, uniqueness: { scope: :target_account_id }
 
   scope :recent, -> { reorder(id: :desc) }
+
+  def local?
+    false # Force uri_for to use uri attribute
+  end
+
+  before_validation :set_uri, only: :create
+
+  private
+
+  def set_uri
+    self.uri = ActivityPub::TagManager.instance.generate_uri_for(self) if uri.nil?
+  end
 end