Allow insertedAt to be null in RemoteRetro component
Allow insertedAt to be null in RemoteRetro component

In attempting to fix a linting issue, I set the `insertedAt` prop as required, but it turns out that it is sometimes null. Defaulting it to null allows it to function as normal and placates the linter.

stride-nyc/remote_retro,stride-nyc/remote_retro,tnewell5/remote_retro,tnewell5/remote_retro,samdec11/remote_retro,stride-nyc/remote_retro