asset_url: Deduplicate assets by content (checksum)

[dkarlovi]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no
Branch?	master

Currently, we allow to deduplicate by origin URL (meaning, if it's from the same URL, it will link to a pre-existing asset).

Another usecase we found is: exactly the same asset used multiple (many) times because of limitations by the origin system (Magento). We could deduplicate by MD5 checksum too, meaning if we pull the same bytes from different URLs, we'd still link to the pre-existing asset, regardless of the URL it was pulled from. We could likely add the new URL to the asset so any URL is later deduplicated via the pre-existing URL method.

[dkarlovi]

Fixed in #373.