Turn monthly_cleanup into a storage maintenance job

From https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/dynamic-storage.html, and assuming that #682 (closed) has been completed first:

• Rename debusine-admin monthly_cleanup to debusine-admin vacuum_storage, in preparation for it becoming a more general storage maintenance job. Run it daily rather than monthly. (!1610 (merged))
• In debusine-admin vacuum_storage, handle files that do not have a local path. (!1626 (merged), !1635 (merged))
• Update debusine-admin vacuum_storage to copy or move files in a scope to other stores as needed to satisfy the populate (!1659 (merged)) and drain (!1660 (merged)) policies and the store-level soft_max_size and max_size limits (!1662 (merged)).

We decided to defer handling the per-scope soft_max_size policy; I filed #728 for that.

Report time in #539 (closed).