Standardize/simplify LESS @imports. Bug 1108664.

- Also make sure lib.less outputs nothing.
Update platform detect regex to account for Windows NT 10.0.
Bug 1111042.
Merge pull request #2589 from jpetto/bug-1111042-ua-sniffing-windows-10

Update platform detect regex to account for Windows NT 10.0.
[fix bug 1111656] Override tabzilla update notification on /plugincheck
Merge pull request #2590 from alexgibson/bug-1111656-override-tabzilla-plugincheck

[fix bug 1111656] Override tabzilla update notification on /plugincheck
Merge pull request #2573 from jpetto/clean-less-imports

Standardize/simplify LESS @imports.