Generate correct image_url for Items from external sources

[Libisch]

• Data scraped from Bakgnowka was converted to photoUnits docs
• The images are stored in AWS, apposed to BHP images which are stored in gcloud.
• Currently all image_urls are automaticaly assigned to google storage:
  https://storage.googleapis.com/bhs-flat-pics/[PicturId].jpg

Expected
API endpoints should identify this and assign the correct url for 'main_image_url':
https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/[_PicturId_].jpg

[Libisch]

@OriHoch Maybe we should change just the frontend (which is where the image_url is currently generated), and wait with bhs_api endpoints until clearmash is integrated?
Anyway, I'll need some help from @inna because testing changes couldn't be preformed without changing the frontend (#351).

As to the items, they are fit and ready, except for the images (locally):
(1/2) Search results:

(2/2) Item page:

[Libisch]

Done. API returns correct url:

[TheGrandVizier]

Waiting for itempage 404 to be fixed.