Merge pull request #975 from ddurnev/master

[RJA-2312][fix]: "Mark as Destroyed" menu item is disabled
[RJA-2301][feature] - Hypermedia links to attachments collection
Merge pull request #976 from rsvato/master

[RJA-2301][feature] - Hypermedia links to attachments collection
[RJA-2372][fix]: Made file attachments serializable
to be possible to send them in remote action result(from agent to backend).