Refactor asset-management (backend)

[tbergmueller]

In #55 we started to serve files directly from the filesystem. This is in general fine, because asset-hashes are keccak256 and it is guessing or brute-force iterating is therefore not feasible.

However, this leaves legacy and unused code;

• In DB, column assets.file_path is no longer used. It shall be removed.

We shall also re-consider, whether heavily relying on the filesystem is a good idea or makes us less flexible in the future.