Not possible to promote FLEX in roster / no visual / no option in CSV uploader

[BenBrostoff]

Currently, the CSV uploaders assume the FLEX by just taking the leftover player from eligibile flex positions for NFL:

https://github.com/BenBrostoff/draftfast/blob/master/draftfast/csv_parse/upload.py#L31-L32

There may be scenarios where you want the FLEX to be able to be picked, and also see FLEX in the visual output:

Notice in the above there's no pointer to what position is FLEX, as there are multiple options.

Because there are many ways to form a valid lineup with NFL FLEX but all of them result in the same point totals given the same players, I'm fine with the defaults. That said, if you want to pick a FLEX, you should be able to, and the CSV uploaders should respect it.