refactor: Move MessageFlag(s) to its own file (#5549)

src/common/Channel.hpp

@@ -1,8 +1,8 @@
 #pragma once
 
-#include "common/FlagsEnum.hpp"
 #include "controllers/completion/TabCompletionModel.hpp"
 #include "messages/LimitedQueue.hpp"
+#include "messages/MessageFlag.hpp"
 
 #include <magic_enum/magic_enum.hpp>
 #include <pajlada/signals/signal.hpp>
@@ -17,8 +17,6 @@ namespace chatterino {
 
 struct Message;
 using MessagePtr = std::shared_ptr<const Message>;
-enum class MessageFlag : int64_t;
-using MessageFlags = FlagsEnum<MessageFlag>;
 
 enum class TimeoutStackStyle : int {
     StackHard = 0,