Upgrade to LESS 3.9.0

Review Request #10427 — Created Feb. 22, 2019 and submitted — Latest diff uploaded

Information

Djblets
release-1.0.x
d92fae8...

Reviewers

This change upgrades us to the latest LESS version for the Djblets
1.0/Review Board 3.0 series. This matches a change which was already
done for 2.0/4.0.

  • Verified that stylesheets still compiled correctly.
  • Built the djblets package.