SD-365 : Spike: Research async notifications

SD-365 : Spike: Research async notifications

First draft.
fix(archive): generate guid on server when creating items

SD-444
SD-365 : Spike: Research async notifications

Finalized with time based notifications pushing.
fix(celery): make sure it returns int for file nums
Audio and video metadata extraction
Merge remote-tracking branch 'gabriel/master'
fix(notifications): start timer only after there are some changes

just to fix the tests, not sure if it's viable
fix(settings): re-enable superdesk.amazon
Spike endpoints
Refactor the endpoint names to *Model
Try to make all endpoints compatible with default Eve apps.
Fix failing tests