Rest backend: Attachment creation, property modification & save

* For the moment, attachments have a single parent
* The attachment is posted when it's saved, then it gets its ID
* Messages have a list with the IDs of their children. When a message is saved for the first time, it gets its ID, then it updates its attachments' parent ID.
* Neither message nor attachment save is implemented for existing objects. This means that every time an attachment is saved, a 'post' operation is called.