Implement the synchronization of promoted releases

This (rather large) merge request implements the synchronization of promoted releases using the new entrypoint arch-release-sync.

The configuration has been extended to allow overriding the local synchronization directory (and other options) on a per project basis and to ensure that each release type name is unique. A Unix timestamp (i.e. date +%s) may be written to a configurable file upon changes to the sync dir. The temporary directory to which artifacts are downloaded may be set to the sync dir (else it is the per-user temporary directory).

Fixes #12 (closed)