Importers enhancements

[vala]

Excel imports are often incomplete or broken.

I think that some kind of data validity preview before running the actual import could be useful.
We could for example do the following :

• Run the import with some kind of "Dry-run" mode
• While importing rows, record errors with a clean and easy API
• Preview import data and errors
• Validate the import

Additionnaly, we could make importers sidekiq friendly to use with sidekiq-job_monitor and allow sidekiq ready apps to always run imports in background.

I think that this can be achieved without too much work regarding the tools we have at our disposal.

What you think @gabriel-cardoso ?

[vala]

A new import system has been integrated, it does not cover all the expressed needs here but works in most cases :

• Handles errors reporting row by row errors
• Runs asynchronously
• Displays a progress bar
• Integrated with the Para's generic job system