Add configuration[statuses][characters_reserved_per_url] to /api/v2/instance

Fixes #3250

changelog.d/characters_reserved_per_url.add

@@ -0,0 +1 @@
+Add configuration[statuses][characters_reserved_per_url] to /api/v2/instance (https://docs.joinmastodon.org/entities/Instance/#characters_reserved_per_url)

lib/pleroma/web/api_spec/operations/instance_operation.ex

@@ -285,6 +285,11 @@ defp instance2 do
               type: :object,
               description: "A map with poll limits for local statuses",
               properties: %{
+                characters_reserved_per_url: %Schema{
+                  type: :integer,
+                  description:
+                    "Each URL in a status will be assumed to be exactly this many characters."
+                },
                 max_characters: %Schema{
                   type: :integer,
                   description: "Posts character limit (CW/Subject included in the counter)"

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

@@ -213,6 +213,7 @@ defp configuration do
 
   defp configuration2 do
     configuration()
+    |> put_in([:statuses, :characters_reserved_per_url], 0)
     |> Map.merge(%{
       urls: %{
         streaming: Pleroma.Web.Endpoint.websocket_url(),