Add helper to determine if a file is an avatar preview

[dpi]

Use a keyvalue to map relationship from file entity -> avatar preview entity ids. This is to improve performance of ID lookups used by avatars_file_storage_load().

This issue will also resolve issues surfaced by #28

[dpi]

Changed from using key store to recycling the file.usage service, which makes use of database. This service' backend can be overridden, or the internals of the new helper can be re-written, if performance of this helper is a drag.

[dpi]

Leaving open for feedback