Renew Gitter SSL cert (2019)

The current *.gitter.im SSL/TLS cert expires on 2019-11-15

Instructions for renewing: https://gitlab.com/gitlab-com/gl-infra/gitter-infrastructure#update-sslhttps-certs

Last time we renewed the SSL cert: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5299

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• https://gitter.pagerduty.com/incidents/PPP1ITV
• https://gitter.pagerduty.com/incidents/P2OS5RV
• https://gitter.pagerduty.com/incidents/P62XXX8
• https://gitter.pagerduty.com/incidents/PNG9QPQ

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cc @viktomas