Use new installer feature from packager.io

Postinstall for openproject is now in its own addon at https://github.com/pkgr/addon-openproject.
Add addon-legacy-installer
fix firefox watcher icon hover incorrect
read column_sums

In 1513a19 I removed column_sums expecting it to be unnecessary. Turns out it is not.
Merge pull request #2047 from opf/fix/wp_column_sums
Merge pull request #2000 from crohr/dev

Use new installer feature from packager.io
Merge pull request #2045 from 0xF013/feature/15418-firefox-watcher-icon-hover-incorrect

fix firefox watcher icon hover incorrect