Standardize/simplify LESS @imports. Bug 1108664.

- Also make sure lib.less outputs nothing.
[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
Update Rust GitHub link
Merge pull request #2573 from jpetto/clean-less-imports

Standardize/simplify LESS @imports.
Merge pull request #2591 from sanxiyn/patch-1

Update Rust GitHub link