[fix bug 1067281] Remove duplicate CSS files from bedrock pages

- /mission/
- /about/powered-by/
- /firefox/channel/
- /firefox/os/devices/
- /about/manifesto/
- /plugincheck/
- /privacy/ base template
- tabzilla.less imported lib.less twice
Merge pull request #2278 from alexgibson/bug-1067281-remove-duplicate-css-files-templates

[fix bug 1067281] Remove duplicate CSS files from bedrock pages