Fixes a bug with the display of the fixed userinfo panel
when the left-nav is open. It now behaves the same as the article
content - squishing at large screen sizes and clipping on small
screens.

Also fixes weird white box bug in Chrome and Safari.

Also consolidating css for #mw-mf-diffview #mw-mf-userinfo.

Depends on having the Echo and Thanks extensions installed.

Dependency: Ibc6916640840b5c1f05297f8f05ee44746f37b9d
Change-Id: I41ef2e80001ae3eb8fd012d554430076b3bc0e3d
Merge "Fixes a bug with the display of the fixed userinfo panel when the left-nav is open. It now behaves the same as the article content - squishing at large screen sizes and clipping on small screens."