Add a clearer uploading state to the video block

[talldan]

What problem does this address?

When uploading a video to the video block, the uploading state isn't particularly clear if the network connection is slow.

There's an initial loading spinner, then the video appears to be uploaded, but sometime later the spinner shows again just before the upload completes.

What is your proposed solution?

It'd be good to replicate what was done in the cover block for videos:
#25401