pkgsummary.py: Add support for global merge

Add a JSON read function, and a summ merge function, with are needed
for merging the section summaries into a global summary.
piuparts-report.py: generate_global_summary()

At the end of processing, collect the section summary JSON files, and
consolidate the information into a single global JSON. Add a special
'overall' reporting-section, holding the 'worst' test result across
all of the ohter reporting-sections.
Add blocking count to the JSON summary format

It is zero for all but 'F'ailed records. Shows the numb4er of binary
packages which cannot be tested because of the failure. The number
used is the highest for all tests reflecting the source package and
reporting-section, regardless of which reporting-section url is
used.
pkgsummary.py: tooltip() routing for DDPO
changelog: Entry for summary.json