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/block.rb

@@ -8,6 +8,7 @@
 #  updated_at        :datetime         not null
 #  account_id        :bigint(8)        not null
 #  target_account_id :bigint(8)        not null
+#  uri               :string
 #
 
 class Block < ApplicationRecord
@@ -19,7 +20,12 @@ class Block < ApplicationRecord
 
   validates :account_id, uniqueness: { scope: :target_account_id }
 
+  def local?
+    false # Force uri_for to use uri attribute
+  end
+
   after_commit :remove_blocking_cache
+  before_validation :set_uri, only: :create
 
   private
 
@@ -27,4 +33,8 @@ class Block < ApplicationRecord
     Rails.cache.delete("exclude_account_ids_for:#{account_id}")
     Rails.cache.delete("exclude_account_ids_for:#{target_account_id}")
   end
+
+  def set_uri
+    self.uri = ActivityPub::TagManager.instance.generate_uri_for(self) if uri.nil?
+  end
 end