Default last_fetched_at to 0 for old clients

Users who have not refreshed their browser tab will poll GitLab using
outdated JS. This change makes the server fall back to the old behavior
(send all comments) for old clients, instead of throwing an exception
for old clients.

app/finders/notes_finder.rb

@@ -4,7 +4,8 @@ class NotesFinder
   def execute(project, current_user, params)
     target_type = params[:target_type]
     target_id   = params[:target_id]
-    last_fetched_at = Time.at(params.fetch(:last_fetched_at).to_i)
+    # Default to 0 to remain compatible with old clients
+    last_fetched_at = Time.at(params.fetch(:last_fetched_at, 0).to_i)
 
     notes = case target_type
     when "commit"