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: Use "web-host" for summary URL

The new summary.json code was using "master-host". For
p.d.o, that is not valid for the web site. Create a new "web-host"
configuration parameter, and use that for the URLs.