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.

db/migrate/20180416210259_add_uri_to_relationships.rb

@@ -0,0 +1,7 @@
+class AddUriToRelationships < ActiveRecord::Migration[5.2]
+  def change
+    add_column :follows, :uri, :string
+    add_column :follow_requests, :uri, :string
+    add_column :blocks, :uri, :string
+  end
+end