Moved getRandomColor method to util/Helpers.cpp (#2974)

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

src/util/Helpers.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <QColor>
 #include <QString>
 
 namespace chatterino {
@@ -17,4 +18,6 @@ QString localizeNumbers(const int &number);
 
 QString kFormatNumbers(const int &number);
 
+QColor getRandomColor(const QString &userId);
+
 }  // namespace chatterino