CI: Create scheduled pipeline

[mweidling]

The repos.json file should be updated periodically so that the webapp is up to date.

For this we have to

• [x] define how often the JSON should update
• [x] create a config for CircleCI
• [x] enable CI/CD in this repo (if not enabled by default)
• [x] let a bot recommit the repos.json periodically

[mweidling]

The update should be done on a daily basis.

[mweidling]

Important/memo: Scheduled workflows will phase out by the end of 2022 in CircleCI. We should use scheduled pipelines instead.

[mweidling]

See also https://support.circleci.com/hc/en-us/articles/360018860473-How-to-push-a-commit-back-to-the-same-repository-as-part-of-the-CircleCI-job.