FIX(server, client): Remove "Write" ACL parent channel inheritance

Since 2a9dcfd and 62b1536 the Mumble server
would overwrite the current channel Write ACL, if the user
had Write ACL permission in the parent channel.
Supposedly, this was done because otherwise malicious users
could create temporary "ungovernable" channels by locking admins out
denying Write ACL for them.
However, this makes ACL management a lot less intuitive with regard
to the Write permission.

This commit reverts those commits and instead adds a check to see
if the user has Write permission in the root channel instead.
The reasoning being: If the server owner grants Write ACL on root,
they probably want those users to be able to moderate every channel.
If instead the server owner only grants Write on part of the channel
tree, normal ACL rules apply and users may lock other users out for
whatever reason.

src/mumble/MainWindow.cpp

@@ -2560,20 +2560,6 @@ void MainWindow::updateMenuPermissions() {
 		target.channel->uiPermissions = p;
 	}
 
-	Channel *cparent = target.channel ? target.channel->cParent : nullptr;
-	ChanACL::Permissions pparent =
-		cparent ? static_cast< ChanACL::Permissions >(cparent->uiPermissions) : ChanACL::None;
-
-	if (cparent && !pparent) {
-		Global::get().sh->requestChannelPermissions(cparent->iId);
-		if (cparent->iId == 0)
-			pparent = Global::get().pPermissions;
-		else
-			pparent = ChanACL::All;
-
-		cparent->uiPermissions = pparent;
-	}
-
 	ClientUser *user           = Global::get().uiSession ? ClientUser::get(Global::get().uiSession) : nullptr;
 	Channel *homec             = user ? user->cChannel : nullptr;
 	ChanACL::Permissions homep = homec ? static_cast< ChanACL::Permissions >(homec->uiPermissions) : ChanACL::None;
@@ -2609,7 +2595,7 @@ void MainWindow::updateMenuPermissions() {
 
 	qaChannelAdd->setEnabled(p & (ChanACL::Write | ChanACL::MakeChannel | ChanACL::MakeTempChannel));
 	qaChannelRemove->setEnabled(p & ChanACL::Write);
-	qaChannelACL->setEnabled((p & ChanACL::Write) || (pparent & ChanACL::Write));
+	qaChannelACL->setEnabled((p & ChanACL::Write) || (Global::get().pPermissions & ChanACL::Write));
 
 	qaChannelLink->setEnabled((p & (ChanACL::Write | ChanACL::LinkChannel))
 							  && (homep & (ChanACL::Write | ChanACL::LinkChannel)));

src/murmur/Messages.cpp

@@ -1756,8 +1756,15 @@ void Server::msgACL(ServerUser *uSource, MumbleProto::ACL &msg) {
 	if (!c)
 		return;
 
-	if (!hasPermission(uSource, c, ChanACL::Write)
-		&& !(c->cParent && hasPermission(uSource, c->cParent, ChanACL::Write))) {
+	// For changing channel properties (the 'Write') ACL we allow two things:
+	// 1) As per regular ACL propagating mechanism, we check if the user has been
+	// granted Write in the channel they try to edit
+	// 2) We allow all users who have been granted 'Write' on the root channel
+	// to be able to edit _all_ channels, independent of actual propagated ACLs
+	// This is done to prevent users who have permission to create (temporary)
+	// channels being able to "lock-out" admins by denying them 'Write' in their
+	// channel effectively becoming ungovernable.
+	if (!hasPermission(uSource, c, ChanACL::Write) && !hasPermission(uSource, qhChannels.value(0), ChanACL::Write)) {
 		PERM_DENIED(uSource, c, ChanACL::Write);
 		return;
 	}