Describe the bug
Since #10545 was merged it's now easy for a user to select a column in a columns block. It's also really easy to turn that selected column into a reusable block.

That reusable block can't really be used anywhere, which perhaps makes it worthless, but also it causes layout issues with the columns (see screenshot).

I'm not sure if this has always been possible or if it's a regression. If the former, I wonder if we need a supports.reusable setting in the block api to prevent this?

To Reproduce
Steps to reproduce the behavior:

1. Add a new columns block
2. Add some content to the columns
3. Open the block navigation popover
4. Select one of the 'column' blocks
5. Using the more menu, add the column to reusable blocks
6. Observe that the columns don't maintain their width (see screenshot)

Expected behavior
Either a) the column should not be allowed to be added as a reusable block, or b) the columns should maintain their widths

Screenshots

Additional context

• Gutenberg 4.1