InstanceView: Expose background image link.

This will make it easier for more clients to support this feature.

config/config.exs

@@ -183,6 +183,7 @@
   email: "example@example.com",
   notify_email: "noreply@example.com",
   description: "A Pleroma instance, an alternative fediverse server",
+  background_image: "/images/city.jpg",
   limit: 5_000,
   chat_limit: 5_000,
   remote_limit: 100_000,

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

@@ -33,6 +33,7 @@ def render("show.json", _) do
       avatar_upload_limit: Keyword.get(instance, :avatar_upload_limit),
       background_upload_limit: Keyword.get(instance, :background_upload_limit),
       banner_upload_limit: Keyword.get(instance, :banner_upload_limit),
+      background_image: Keyword.get(instance, :background_image),
       pleroma: %{
         metadata: %{
           features: features(),

test/web/mastodon_api/controllers/instance_controller_test.exs

@@ -31,7 +31,8 @@ test "get instance information", %{conn: conn} do
              "upload_limit" => _,
              "avatar_upload_limit" => _,
              "background_upload_limit" => _,
-             "banner_upload_limit" => _
+             "banner_upload_limit" => _,
+             "background_image" => _
            } = result
 
     assert result["pleroma"]["metadata"]["features"]