Make upload templates as a flat file so we can have 1 upload template per model

[gab.beck]

It would be interesting to have the possibility to have upload templates as flat file in order to ease the management of upload templates.

This would allow us to have to manage only 1 template, and a list of columns that are "uploadable"

We would need to have as many columns as dimensions, 1 column for the line item and 1 for the value we're trying to upload.

Please find an example attached