Add highlights for first messages (#3267)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/controllers/highlights/HighlightPhrase.hpp

@@ -82,6 +82,7 @@ public:
     static QColor FALLBACK_HIGHLIGHT_COLOR;
     static QColor FALLBACK_REDEEMED_HIGHLIGHT_COLOR;
     static QColor FALLBACK_SUB_COLOR;
+    static QColor FALLBACK_FIRST_MESSAGE_HIGHLIGHT_COLOR;
 
 private:
     QString pattern_;