Provide contrast between processed and un-processed items in the message list

A CSS rule is added to de-emphasize translated (and proofread) messages
from messages that need user action (untranslated or fuzzy).

The hover color has been modified to avoid interfering with the color coding.

Change-Id: I093f6145c9d278842d589ce8d62fb66c2e54d9e5
Merge "Provide contrast between processed and un-processed items in the message list"