Consider adding UTF-8 BOM to CSV exports

I'm opening this issue so we have a record that this was considered but I'm currently leaning towards doing nothing.

CSV exports currently do not include [UTF-8 byte-order-marks](https://en.wikipedia.org/wiki/Byte_order_mark). In general, this is a good practice because "the Unicode Standard permits the BOM in UTF-8 but does not require or recommend its use." (https://en.wikipedia.org/wiki/Byte_order_mark)

Unfortunately, modern versions of Excel continue to open CSVs that don't have BOMs as ASCII. That regularly trips up users:
- They only have a few UTF-8 characters so don't notice and end up with bad chars in their analysis
- They notice bad chars but are stuck on what to do next
- They somehow find the tip at the bottom of [this docs section](https://docs.getodk.org/central-submissions/#export-options) and are annoyed by that process

If we add the BOM, exported CSVs would open as expected in Excel when double-clicked. However, it's likely that other downstream tools would then have trouble opening our CSVs. The advantage of the current state of things is that there is one known bad behavior. With a BOM, there are likely to be various different kinds of problems that manifest differently.