[openstreetmap/openstreetmap-website] Add In-Reply-To, References headers to email (#2462)

[mmd]

Before changing any of the existing messages (which is a lot of work), I'd like to try two other things:

* Add new header `X-Entity-Ref-ID`  -> set value to `ref`, check if this fixes threading
* Add new header `X-Entity-Ref-ID`  -> set value to some random value to turn off threading in gmail altogether


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https://github.com/openstreetmap/openstreetmap-website/pull/2462#issuecomment-570057149
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