UoE: Datashare fixes on top of 8.3 upgrade (PRs #11-#20)

[milanmajchrak]

Fixed:

• Missing dc.date.available
• Generation/regeneration of the ZIP file
• The Download all files button is visible even when the user does not have permission to download a file
• The dc.subject.ddc metadata value is exposed via OAI-PMH, but not in the coverage field
• Temporal metadata fields are not loaded after the page is reloaded or when someone else is editing the item
• Subject selection from a value tree, as in version 6
• The Description metadata field is mandatory