[mediawiki.action.history.diff.css] Revert 1.19 style changes back to how it was...

[mediawiki.action.history.diff.css] Revert 1.19 style changes back to how it was in REL1_18
* After the 1.19wmf1 deployment several people have complained about various aspects of the new diff styling
-- The contrast being too low in the highlighted part (darker background behind bolded text)
-- The colors not being obvious perhaps (orange/blue)
-- Color blind users not seeing the difference very well between the light tones of the orange and blue
-- Trevor mentioned something about W3C Accessibility guidelines

To play it safe for now I think we should revert these changes to the status quo, and take the next few days (or weeks) to carefully check the concerns, perhaps look at other diff tools out there for inspiration (GitHub, Gerrit, LocalWiki, ..).

* Goes back to r91762 (last rev before REL1_18)
-- reverting: r94429, r94461, r101147, r105280, r106884, r107127, r108975, r109932
* Re-opens bug 33335