Copy emoji in the subject from parent post

Sometimes people put emoji in the subject, which results in the subject
looking broken if someone replies to it from a server that does not
have the said emoji under the same shortcode. This patch solves the problem
by extending the emoji set available in the summary to that of the parent
post.

test/fixtures/tesla_mock/emoji-in-summary.json

@@ -0,0 +1,49 @@
+{
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://patch.cx/schemas/litepub-0.1.jsonld",
+    {
+      "@language": "und"
+    }
+  ],
+  "actor": "https://patch.cx/users/rin",
+  "attachment": [],
+  "attributedTo": "https://patch.cx/users/rin",
+  "cc": [
+    "https://patch.cx/users/rin/followers"
+  ],
+  "content": ":joker_disapprove: <br><br>just grabbing a test fixture, nevermind me",
+  "context": "https://patch.cx/contexts/2c3ce4b4-18b1-4b1a-8965-3932027b5326",
+  "conversation": "https://patch.cx/contexts/2c3ce4b4-18b1-4b1a-8965-3932027b5326",
+  "id": "https://patch.cx/objects/a399c28e-c821-4820-bc3e-4afeb044c16f",
+  "published": "2021-03-22T16:54:46.461939Z",
+  "sensitive": null,
+  "source": ":joker_disapprove: \r\n\r\njust grabbing a test fixture, nevermind me",
+  "summary": ":joker_smile: ",
+  "tag": [
+    {
+      "icon": {
+        "type": "Image",
+        "url": "https://patch.cx/emoji/custom/joker_disapprove.png"
+      },
+      "id": "https://patch.cx/emoji/custom/joker_disapprove.png",
+      "name": ":joker_disapprove:",
+      "type": "Emoji",
+      "updated": "1970-01-01T00:00:00Z"
+    },
+    {
+      "icon": {
+        "type": "Image",
+        "url": "https://patch.cx/emoji/custom/joker_smile.png"
+      },
+      "id": "https://patch.cx/emoji/custom/joker_smile.png",
+      "name": ":joker_smile:",
+      "type": "Emoji",
+      "updated": "1970-01-01T00:00:00Z"
+    }
+  ],
+  "to": [
+    "https://www.w3.org/ns/activitystreams#Public"
+  ],
+  "type": "Note"
+}

test/pleroma/web/common_api_test.exs

@@ -25,6 +25,11 @@ defmodule Pleroma.Web.CommonAPITest do
 
   require Pleroma.Constants
 
+  setup_all do
+    Tesla.Mock.mock_global(fn env -> apply(HttpRequestMock, :request, [env]) end)
+    :ok
+  end
+
   setup do: clear_config([:instance, :safe_dm_mentions])
   setup do: clear_config([:instance, :limit])
   setup do: clear_config([:instance, :max_pinned_statuses])
@@ -517,6 +522,27 @@ defmodule Pleroma.Web.CommonAPITest do
       assert url == "#{Pleroma.Web.base_url()}/emoji/blank.png"
     end
 
+    test "it copies emoji from the subject of the parent post" do
+      %Object{} =
+        object =
+        Object.normalize("https://patch.cx/objects/a399c28e-c821-4820-bc3e-4afeb044c16f",
+          fetch: true
+        )
+
+      activity = Activity.get_create_by_object_ap_id(object.data["id"])
+      user = insert(:user)
+
+      {:ok, reply_activity} =
+        CommonAPI.post(user, %{
+          in_reply_to_id: activity.id,
+          status: ":joker_disapprove:",
+          spoiler_text: ":joker_smile:"
+        })
+
+      assert Object.normalize(reply_activity).data["emoji"][":joker_smile:"]
+      refute Object.normalize(reply_activity).data["emoji"][":joker_disapprove:"]
+    end
+
     test "deactivated users can't post" do
       user = insert(:user, is_active: false)
       assert {:error, _} = CommonAPI.post(user, %{status: "ye"})

test/support/http_request_mock.ex

@@ -1278,6 +1278,15 @@ defmodule HttpRequestMock do
      }}
   end
 
+  def get("https://patch.cx/objects/a399c28e-c821-4820-bc3e-4afeb044c16f", _, _, _) do
+    {:ok,
+     %Tesla.Env{
+       status: 200,
+       body: File.read!("test/fixtures/tesla_mock/emoji-in-summary.json"),
+       headers: activitypub_object_headers()
+     }}
+  end
+
   def get(url, query, body, headers) do
     {:error,
      "Mock response not implemented for GET #{inspect(url)}, #{query}, #{inspect(body)}, #{