Federate emoji as anonymous objects

Usually an id should point to another AP object
and the image file isn’t an AP object. We currently
do not provide standalone AP objects for emoji and
don't keep track of remote emoji at all.
Thus just federate them as anonymous objects,
i.e. objects only existing within a parent context
and using an explicit null id.

IceShrimp.NET previously adopted anonymous objects
for remote emoji without any apparent issues. See:
333611f65e

Fixes: #694

CHANGELOG.md

@@ -17,6 +17,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Fixed
 - Meilisearch: order of results returned from our REST API now actually matches how Meilisearch ranks results
+- Emoji are now federated as anonymous objects, fixing issues with
+  some strict servers e.g. rejecting e.g. remote emoji reactions
 
 ## Changed
 - Refactored Rich Media to cache the content in the database. Fetching operations that could block status rendering have been eliminated.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -951,7 +951,7 @@ defp build_emoji_tag({name, url}) do
       "name" => ":" <> name <> ":",
       "type" => "Emoji",
       "updated" => "1970-01-01T00:00:00Z",
-      "id" => url
+      "id" => nil
     }
   end
 

test/pleroma/web/activity_pub/transmogrifier/note_handling_test.exs

@@ -700,7 +700,7 @@ test "take_emoji_tags/1" do
     assert Transmogrifier.take_emoji_tags(user) == [
              %{
                "icon" => %{"type" => "Image", "url" => "https://example.org/firefox.png"},
-               "id" => "https://example.org/firefox.png",
+               "id" => nil,
                "name" => ":firefox:",
                "type" => "Emoji",
                "updated" => "1970-01-01T00:00:00Z"

test/pleroma/web/activity_pub/views/user_view_test.exs

@@ -43,7 +43,7 @@ test "Renders with emoji tags" do
              "tag" => [
                %{
                  "icon" => %{"type" => "Image", "url" => "/test"},
-                 "id" => "/test",
+                 "id" => nil,
                  "name" => ":bib:",
                  "type" => "Emoji",
                  "updated" => "1970-01-01T00:00:00Z"