expose bubble timeline availability

Signed-off-by: marcin mikołajczak <git@mkljczk.pl>

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

@@ -183,6 +183,9 @@ def features do
       "events",
       "multitenancy",
       "pleroma:bites",
+      if !Enum.empty?(Config.get([:instance, :local_bubble], [])) do
+        "bubble_timeline"
+      end,
       # Akkoma compatibility
       if Pleroma.Language.Translation.configured?() do
         "akkoma:machine_translation"