piuparts-report.py: De-indent global summary func

Remove four spaces of indent from the body of
generate_global_summary().
piuparts-report.py: Refactor global summary

Factor out the section sorting logic to from generate_global_summary()
to another function, sections_by_precedence().
piuparts-report.py: Restore counts.txt to web

Commit 70fe80693 moved the counts.txt file, which drives the section test
result chart, from the web section directory to the master section directory,
making it no longer visible from a web view. This change restores a copy of
the counts.txt file to its original location, in the web section directory.
piuparts-report.py: Link to counts.txt

Below the chart on each section page.