Only support the new Transloadit files_filtered `avatar_thumnails_xxx` template

Only support the new Transloadit files_filtered avatar_thumnails_xxx template

Follow-up to https://dev.gitlab.org/gitlab/gitter/webapp/merge_requests/10 where we supported the old and new template while we shipped the new template out to production

Related to https://gitlab.com/gitlab-org/gitter/webapp/issues/2192

Testing strategy

• Tested with avatar_thumbnails_dev, https://transloadit.com/templates/edit/619863e049a411e6ab64ebd07f7ad0e3
• Setup a tunnel to the outside world. I used ngrok
  • ngrok http 5000
  • Copy the URL it spits out, add /api at the end of it and use it as the apiBasePath here in transloadit-signature.js so the notify_url is accessible
• Copy secrets from Gitter dev secrets to your own .env
• Re-evaluate the .env secrets and restart the Gitter server
• Create a community and upload an avatar (see it succeed)