StatusView: implement pleroma.context field

This field replaces the now deprecated conversation_id field, and now
exposes the ActivityPub object `context` directly via the MastoAPI
instead of relying on StatusNet-era data concepts.

lib/pleroma/web/api_spec/schemas/status.ex

@@ -142,9 +142,15 @@ defmodule Pleroma.Web.ApiSpec.Schemas.Status do
             description:
               "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`"
           },
+          context: %Schema{
+            type: :string,
+            description: "The thread identifier the status is associated with"
+          },
           conversation_id: %Schema{
             type: :integer,
-            description: "The ID of the AP context the status is associated with (if any)"
+            deprecated: true,
+            description:
+              "The ID of the AP context the status is associated with (if any); deprecated, please use `context` instead"
           },
           direct_conversation_id: %Schema{
             type: :integer,
@@ -319,6 +325,7 @@ defmodule Pleroma.Web.ApiSpec.Schemas.Status do
       "pinned" => false,
       "pleroma" => %{
         "content" => %{"text/plain" => "foobar"},
+        "context" => "http://localhost:4001/objects/8b4c0c80-6a37-4d2a-b1b9-05a19e3875aa",
         "conversation_id" => 345_972,
         "direct_conversation_id" => nil,
         "emoji_reactions" => [],