refactor(organization): switch to one-bucket-per-org policy

[RonanMorgan]

It seems to me that it's a bit more secure and the images will be simpler to gather for data scientist if we save all the images of the same organization in one bucket. Furthermore I have added the camera_id in the name of the picture.

[frgfm]

With the recent updates, here is what the PR does:

• switch from "one bucket for all" strategy to "one bucket per organization"
• cleans S3 storage interactions (check later Switch from boto3 to async version (aioboto3) #362)
• add proper storage initialization in db.py
• update tests accordingly

[frgfm]

cf. #304