Allow blocking invites from people you don't share a room with

Matrix currently has a significant moderation loophole, thanks to
invites. Right now, anyone can invite anyone to a room - and clients
like NeoChat will gladly display these rooms to them and even give you
a notification.

However, this creates a pretty easy attack since room names and avatars
are arbitrary and this is a known vector of harassment in the Matrix
community. There's currently no tools to block this server-side, so
let's try to improve the situation where we can.

This adds a new setting to the Security page, wherein it allows you to
block invites from people you don't share a room with. This prevents the
notification from appearing and NeoChat will attempt to leave the room
immediately.

Since this depends on MSC 2666 - a currently unstable feature - the
server may not support it and NeoChat will disable the setting in this
case.

src/neochatconnection.h

@@ -79,6 +79,11 @@ class NeoChatConnection : public Quotient::Connection
      */
     Q_PROPERTY(bool isOnline READ isOnline WRITE setIsOnline NOTIFY isOnlineChanged)
 
+    /**
+     * @brief Whether the server supports querying a user's mutual rooms.
+     */
+    Q_PROPERTY(bool canCheckMutualRooms READ canCheckMutualRooms NOTIFY canCheckMutualRoomsChanged)
+
 public:
     /**
      * @brief Defines the status after an attempt to change the password on an account.
@@ -95,6 +100,7 @@ public:
 
     Q_INVOKABLE void logout(bool serverSideLogout);
     Q_INVOKABLE QVariantList getSupportedRoomVersions() const;
+    bool canCheckMutualRooms() const;
 
     /**
      * @brief Change the password for an account.
@@ -196,6 +202,7 @@ Q_SIGNALS:
     void passwordStatus(NeoChatConnection::PasswordStatus status);
     void userConsentRequired(QUrl url);
     void badgeNotificationCountChanged(NeoChatConnection *connection, int count);
+    void canCheckMutualRoomsChanged();
 
 private:
     bool m_isOnline = true;
@@ -208,4 +215,6 @@ private:
     int m_badgeNotificationCount = 0;
 
     QHash<QUrl, LinkPreviewer *> m_linkPreviewers;
+
+    bool m_canCheckMutualRooms = false;
 };