Hide/delete page via the admin panel

[AlexandreDubray]

Currently, we can delete file via the admin panel and check the box "Also delete the files related to this content on the file system.". However I do not see a way to re-add a file that already exists to the syllabus, is there one ?

Also I think that we should have two options for "removing" a file from the syllabus:

1. Removing it definitely, which mean that it is not kept on the file system
2. Hide it, which only remove it from the syllabus (but it would still be shown in the admin panel)