Allow blocks to be dropped in invalid locations

[LaCuneta]

(I have not decided if this is the right strategy or not yet, but am creating the issue for consideration.)

At the moment, invalid blocks cannot be dropped into a chain and the UI highlights the location a block is allowed when a drag starts. The disallowed locations are usually because block tags restrict it, and that is usually by agent or breed type. It can also happen when a block is marked "terminal" so no blocks can be dropped after it. Some feedback says users find this confusing, and they'd like to drop blocks anywhere. If a block is invalid for a location it could be "low-lighted", indicating that it will not execute, perhaps with a way to discover the reason why it won't work (pop-up code tip).