Re-think the garbage-collection of reports with custom names in Maat

[integraledelebesgue]

Reports with custom names often belong to the "All" slice. Reports from this slice are garbage-collected once in a week, which is sometimes too short.

We need to think how to handle them more conveniently. Options are:

1. Extend the garbage collection period (e.g. to 1 month)
2. Allow specifying the report's lifetime manually on experiment dispatch
3. Introduce a dedicated slice (e.g. "Custom") with a separate GC policy