Rebase on Upstream

This issue is a meta issue to track questions related with rebasing our current codebases back on Upstream.

Related upstream issues are:

• Plugin System (open since 29.12.2011 !!!)
  https://github.com/frab/frab/issues/10
• Make event types configurable (18.03.2017)
  https://github.com/frab/frab/issues/302
  • configurable event types per conference (opened 03.09.2019 - merged 25.10.2019)
    https://github.com/frab/frab/pull/530
• README.PaaS.md should require a persistent storage (open since 06.03.2019)
  https://github.com/frab/frab/issues/466