Bug 778033

* src/client/conversation-viewer/conversation-message.vala
  (ConversationMessage::highlight_search_terms): Also check the subject
  for matching items and highlight if found.

* src/client/conversation-viewer/conversation-list-box.vala
  (ConversationListBox::EmailRow): Update matching row class to
  disambiguate from matching headers.

* ui/geary.css: Ensure matching subject labels get highlighted.