Server: Find asset files through one function, first data path winning

[lehni]

Follow-up to #385, which was merged before these review fixes were pushed.

• Model._getAssetFiles() walks a record's asset files location by location, and the first declared data path wins. The counts on writes, getAssetReferences(), loading (_forEachAssetFile()) and signing (_mapAssetFiles()) all use it, so the ownership rule exists in one place instead of two.
• Loaded files at places that overlapping data paths reach get the first path's storage. Before, the last one won, contrary to what Server: Count each asset location once with overlapping data paths #385's changelog and docs say.
• Arrays nested inside file arrays ([a, [b]]) were walked recursively when loading. Now only one level is, as counts and references already did. Asset schemas can't produce nested arrays.
• Tests: overlapping data paths on the same storage (counts, references, recount), and the storage of loaded files. The changelog entry of Server: Count each asset location once with overlapping data paths #385 is updated.