fix resolution of GTS user keys

lib/pleroma/signature.ex

@@ -10,17 +10,14 @@ defmodule Pleroma.Signature do
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.ActivityPub
 
+  @known_suffixes ["/publickey", "/main-key"]
+
   def key_id_to_actor_id(key_id) do
     uri =
-      URI.parse(key_id)
+      key_id
+      |> URI.parse()
       |> Map.put(:fragment, nil)
-
-    uri =
-      if not is_nil(uri.path) and String.ends_with?(uri.path, "/publickey") do
-        Map.put(uri, :path, String.replace(uri.path, "/publickey", ""))
-      else
-        uri
-      end
+      |> remove_suffix(@known_suffixes)
 
     maybe_ap_id = URI.to_string(uri)
 
@@ -36,6 +33,16 @@ defmodule Pleroma.Signature do
     end
   end
 
+  defp remove_suffix(uri, [test | rest]) do
+    if not is_nil(uri.path) and String.ends_with?(uri.path, test) do
+      Map.put(uri, :path, String.replace(uri.path, test, ""))
+    else
+      remove_suffix(uri, rest)
+    end
+  end
+
+  defp remove_suffix(uri, []), do: uri
+
   def fetch_public_key(conn) do
     with %{"keyId" => kid} <- HTTPSignatures.signature_for_conn(conn),
          {:ok, actor_id} <- key_id_to_actor_id(kid),