Unique identifier for prediction events

[glaubervila]

Add a new field to the predictions table that is a unique identifier. This field will allow the creation of unique URLs for events, replacing the ids

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1. The unique identifier must be a hash formed by joining three identifiers, the asteroid, the star and the time of the prediction.
2. This hash should allow the event to be updated when the prediction is reprocessed.
3. It is interesting to have a pair of fields to inform the event creation date and the last update date.
4. Importing events should no longer delete pre-existing events, you should do a COPY ... On Conflict UPDATE using the new identifier.