Logic for data editing: validation/correction/de-duplication

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As pointed out in issues #222, #229, #238, we might need an editing logic in our data ingestion. Inspired by these, some things would be nice to have:

• de-duplicate events (e.g. same events coming in from multiple sources)
• check if fields might not be valid
• clear events to be shown in public places (a.k.a. safe-for-kids check)
• edit events that have obvious errors

I am starting to sketch ideas for a pipeline in this pad. Please comment, and add your thoughts.

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FYI, @tomsrocket @jdemaeyer