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.cpp

@@ -25,6 +25,7 @@
 
 #include <Quotient/csapi/content-repo.h>
 #include <Quotient/csapi/profile.h>
+#include <Quotient/csapi/versions.h>
 #include <Quotient/database.h>
 #include <Quotient/jobs/downloadfilejob.h>
 #include <Quotient/qt_connection_util.h>
@@ -132,6 +133,21 @@ void NeoChatConnection::connectSignals()
         Q_EMIT homeNotificationsChanged();
         Q_EMIT homeHaveHighlightNotificationsChanged();
     });
+
+    // Fetch unstable features
+    // TODO: Expose unstableFeatures() in libQuotient
+    connect(
+        this,
+        &Connection::connected,
+        this,
+        [this] {
+            auto job = callApi<GetVersionsJob>(BackgroundRequest);
+            connect(job, &GetVersionsJob::success, this, [this, job] {
+                m_canCheckMutualRooms = job->unstableFeatures().contains("uk.half-shot.msc2666.query_mutual_rooms"_ls);
+                Q_EMIT canCheckMutualRoomsChanged();
+            });
+        },
+        Qt::SingleShotConnection);
 }
 
 int NeoChatConnection::badgeNotificationCount() const
@@ -200,6 +216,11 @@ QVariantList NeoChatConnection::getSupportedRoomVersions() const
     return supportedRoomVersions;
 }
 
+bool NeoChatConnection::canCheckMutualRooms() const
+{
+    return m_canCheckMutualRooms;
+}
+
 void NeoChatConnection::changePassword(const QString &currentPassword, const QString &newPassword)
 {
     auto job = callApi<NeochatChangePasswordJob>(newPassword, false);