Ensure consistency between download modes

[Facyla]

Downloading files and folders from Linshare has several differences and issues: the objective would be to ensure that users experiment the same behaviour when downloading

1. obtaining an archive or not
2. grouped files/folders or not,
3. content is downloaded or not (some files open in the browser)
4. folders structure is not kept

For now:

• downloading single file => file download is sent as is, except PDF files which are opened in the browser
• downloading multiple files => each file download is sent as is
• downloading a single folder => download is sent as compressed file (as ZIP)
• downloading several folders => each folder download is sent as compressed file (as ZIP)
• download folders archive do not contain subfolders that do not have files in it, which is really annoying when one wants to share eg. a structure template, without actual content in it