[feature] `files` column should be a table ?

[Azecko]

This is a big issue that requires a lot of changes in the UI/UX and also in the database.

Here is the thing :
Many users have multiple files when registering the exam to print. For example, a file with the whole exam that is 1000+ pages, and one file that is just the questions for the exam. That is maybe only 8 pages.

It would be great to be able to dissociate how each files are treated. Example :
The file My_Exam_1000pages.pdf should be printed as A4, Greyscale, Recto-Verso
The file My_Questions_8pages.pdf should be printed as A3, Colored, Recto-Verso

While still being in only one entry.

That means that some columns will have to move out to a new table, maybe named lines, this concerns following columns :

• files (that should now be one entry per file)
• exam_pages (becoming the number of pages of the document)
• exam_students (number of copies for the document)
• paper_format (printed in A4/A3)
• paper_color (greyscale/color)
• need_scan (0/1)
• print (recto-verso/recto)
• price_unit (float, default 0)
• price_total (float, default 0)

Each file that is registered should be a new entry in the lines table.

Meaning changes in : Registering form, Modal, printing duration calculation, ...