Change the way Twitch accounts are stored in AccountManager

This is done in a way which should simplify abstracting it to other
types of accounts if needed in the future

Remove comment about removing singletons - we're keeping them (and probably restoring some)

IrcManager now updates its "account" reference automatically through the
AccountManager.Twitch.userChanged-signal

Remove unused IrcManager getUser-method

IrcManager::beginConnecting is no longer called asynchronously. This
might want to be reverted in a more controlled asynchronous manner.

User Accounts are now stored as Shared Pointers instead of using
references/copies everywhere

src/util/urlfetch.hpp

@@ -87,11 +87,18 @@ static void put(QUrl url, std::function<void(QJsonObject)> successCallback)
     auto manager = new QNetworkAccessManager();
     QNetworkRequest request(url);
 
+    auto &accountManager = AccountManager::getInstance();
+    auto currentTwitchUser = accountManager.Twitch.getCurrent();
+    QByteArray oauthToken;
+    if (currentTwitchUser) {
+        oauthToken = currentTwitchUser->getOAuthToken().toUtf8();
+    } else {
+        // XXX(pajlada): Bail out?
+    }
+
     request.setRawHeader("Client-ID", getDefaultClientID());
     request.setRawHeader("Accept", "application/vnd.twitchtv.v5+json");
-    request.setRawHeader(
-        "Authorization",
-        "OAuth " + AccountManager::getInstance().getTwitchUser().getOAuthToken().toUtf8());
+    request.setRawHeader("Authorization", "OAuth " + oauthToken);
 
     NetworkManager::urlPut(std::move(request), [=](QNetworkReply *reply) {
         if (reply->error() == QNetworkReply::NetworkError::NoError) {