Fix crash resulting from a mutex deadlock when switching users

[Nerixyz]

Description

Reproduction

• Open a reply thread popup in a Twitch channel
• Close it
• Switch the current user (make sure no message was sent in that channel after you closed the popup)
• 💥

Why?

1. The reply thread popup creates a virtual channel that displays the messages in that thread:

   chatterino2/src/widgets/dialogs/ReplyThreadPopup.cpp

   Lines 141 to 146 in ca9c91a

   	ChannelPtr virtualChannel;
   	if (sourceChannel->isTwitchChannel())
   	{
   	virtualChannel =
   	std::make_shared<TwitchChannel>(sourceChannel->getName());
   	}

2. This channel is then part of the messageAppended callback:

   chatterino2/src/widgets/dialogs/ReplyThreadPopup.cpp

   Lines 174 to 175 in ca9c91a

   	sourceChannel->messageAppended.connect(
   	[this, virtualChannel](MessagePtr &message, auto) {

3. When you close the popup, the connection is destroyed (~ScopedConnection ⇒ Callback::disconnect ⇒ CallbackBodyBase::disconnect).
4. When you switch the current user, the application reconnects:

   chatterino2/src/providers/twitch/TwitchIrcServer.cpp

   Lines 73 to 75 in ca9c91a

   	getApp()->accounts->twitch.currentUserChanged.connect([this]() {
   	postToThread([this] {
   	this->connect();

   chatterino2/src/providers/irc/AbstractIrcServer.cpp

   Lines 113 to 117 in ca9c91a

   	void AbstractIrcServer::connect()
   	{
   	assert(this->initialized_);
   	this->disconnect();

5. Eventually you'll get disconnected, causing AbstractIrcServer::onDisconnected to be called. This will send a message on all channels:

   chatterino2/src/providers/irc/AbstractIrcServer.cpp

   Lines 346 to 364 in ca9c91a

   	void AbstractIrcServer::onDisconnected()
   	{
   	std::lock_guard<std::mutex> lock(this->channelMutex);
   	MessageBuilder b(systemMessage, "disconnected");
   	b->flags.set(MessageFlag::DisconnectedMessage);
   	auto disconnectedMsg = b.release();
   	for (std::weak_ptr<Channel> &weak : this->channels.values())
   	{
   	std::shared_ptr<Channel> chan = weak.lock();
   	if (!chan)
   	{
   	continue;
   	}
   	chan->addMessage(disconnectedMsg);
   	}
   	}

6. Specifically, it will send a message in the channel you opened the reply popup from.
7. This will invoke the messageAppended signal:

   chatterino2/src/common/Channel.cpp

   Line 111 in ca9c91a

   this->messageAppended.invoke(message, overridingFlags);

8. This will destroy all disconnected callback bodies.
9. Remember the disconnected callback body with our virtual channel from step 3? It will get destroyed here.
10. Since the virtual channel is part of that callback, it will be destroyed. Thus, dropSeventvChannel will be called:

    chatterino2/src/providers/twitch/TwitchChannel.cpp

    Lines 162 to 165 in ca9c91a

    	TwitchChannel::~TwitchChannel()
    	{
    	getApp()->twitch->dropSeventvChannel(this->seventvUserID_,
    	this->seventvEmoteSetID_);

11. This will lock the channelMutex:

    chatterino2/src/providers/twitch/TwitchIrcServer.cpp

    Lines 647 to 655 in ca9c91a

    	void TwitchIrcServer::dropSeventvChannel(const QString &userID,
    	const QString &emoteSetID)
    	{
    	if (!this->seventvEventAPI)
    	{
    	return;
    	}
    	std::lock_guard<std::mutex> lock(this->channelMutex);

    But AbstractIrcServer locked this mutex in step 5.

---

This PR moves the virtual channel to the popup, thus it will be destroyed when the popup is destroyed.