Data import: limit csv file size

[dpalomino]

As the import process done via a .csv file is not executed asynchronously right now, we would need to limit the file size of the .csv import file to avoid too long import executions that will be prone to errors (like user trying to reload the page, getting some server timeout errors, etc).

As an estimation, a .csv file containing 1k records with 35 columns is 500KB. Maybe that could be a good threshold.