don't move a share mount point into a different mount point

fix for #8429 

Don't move a share mount point into another mount point.

cc @PVince81 @icewind1991 
I think this was the consensus in #8429. Often a external storage is also accessed without ownCloud. If people move a file/folder to a mount point they expect that the files are there, also if they access the storage directly. But that's not the case for a mount point. To avoid this confusion I think that it is the best to block it for now, like discussed in the issue.

Remember, the moving mount points is a new feature and we are already deep in feature freeze. If we find good solutions how to allow moving one mount point to another one then we can still implement it for OC8. But removing something for OC8 which was in OC7 because we see that it it not that easy to match the users expectation is quite hard.

TL;DR for now let's go with the solution we know that it will work in all circumstances and which was already discussed. We can always weaken the rule in the feature if we come to the conclusion that in some areas we can give the user more freedom.