Add automatic dependency updates via Renovate

Add automatic dependency updates via Renovate

Fix https://gitlab.com/gitlab-org/gitter/webapp/issues/1880

• Adds new CI variable RENOVATE_TOKEN which has a GitLab access token with api access, https://gitlab.com/gitlab-org/gitter/webapp/-/settings/ci_cd

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• https://renovatebot.com/docs/configuration-options/

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• https://simplabs.com/blog/2019/04/24/dependency-updates-for-gitlab/
• https://medium.com/@mikebarkmin/renovate-your-gitlab-projects-automatically-bbba188669b5
• https://www.froehlichundfrei.de/blog/2018-03-13-howto-use-renovate-and-gitlab-ci-to-keep-your-javascript-dependencies-updated/