Add notifications/webhook capabilties

[pombredanne]

When I create a project I may want tobe notified when the scans are complete either through email or some webhook callback.

[tdruez]

That "subscription" feature exists in the scanner module, we can re-use it on the scanpipe side.
See:

• https://github.com/nexB/scancode.io/blob/main/scanner/models.py#L386
• https://github.com/nexB/scancode.io/blob/main/scanner/models.py#L412