Investments can be reclassified to a different heading during the participatory budget process.
Whilst we are recording this change of heading in the `previous_heading_id` attribute, we are only keeping the _last_ heading. If there are multiple reclassifications we lose this chain of reclassifications.
In this commit we are adding an `original_heading_id` attribute, that will only be set once, when creating the investment, and will not get lost with multiple reclassificaitons of an investment.
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