Add report_dir and report_path functions

This replace the report-dir option.
Add a save_report function
Save report summaries as json files
Make %reports a global variable

This allows to share it between make_reports_index and load_report,
which should avoid loading twice the same report.
make_reports_index: use summaries rather than full reports

When possible, use summaries rather than full reports. Create summary
from the full report if it does not exist yet.
When adding a new report, only update index pages that changed
Only include the 20 latest reports on the index by type