Add `pleroma.in_reply_to_account_acct` to MastoAPI status entity

docs/api/differences_in_mastoapi_responses.md

@@ -20,6 +20,7 @@ Has these additional fields under the `pleroma` object:
 
 - `local`: true if the post was made on the local instance.
 - `conversation_id`: the ID of the conversation the status is associated with (if any)
+- `in_reply_to_account_acct`: the `acct` property of User entity for replied user (if any)
 - `content`: a map consisting of alternate representations of the `content` property with the key being it's mimetype. Currently the only alternate representation supported is `text/plain`
 - `spoiler_text`: a map consisting of alternate representations of the `spoiler_text` property with the key being it's mimetype. Currently the only alternate representation supported is `text/plain`
 

lib/pleroma/web/mastodon_api/views/status_view.ex

@@ -238,6 +238,7 @@ def render("status.json", %{activity: %{data: %{"object" => _object}} = activity
       pleroma: %{
         local: activity.local,
         conversation_id: get_context_id(activity),
+        in_reply_to_account_acct: reply_to_user && reply_to_user.nickname,
         content: %{"text/plain" => content_plaintext},
         spoiler_text: %{"text/plain" => summary_plaintext}
       }

test/web/mastodon_api/status_view_test.exs

@@ -128,6 +128,7 @@ test "a note activity" do
       pleroma: %{
         local: true,
         conversation_id: convo_id,
+        in_reply_to_account_acct: nil,
         content: %{"text/plain" => HtmlSanitizeEx.strip_tags(note.data["object"]["content"])},
         spoiler_text: %{"text/plain" => HtmlSanitizeEx.strip_tags(note.data["object"]["summary"])}
       }