Custom blocks cannot be properly exported and imported

Configuration management shouldn't export custom blocks as currently it will result in broken block.

A custom block is made of two entities, one for the placement and one for the actual content. Only the actual placement can be exported with cmi. The content can not.
Therefore this will result in "Block description Broken/Missing" error on site where the config is imported. And since there is no option to disable custom blocks from being exported through Configuration management this will brake the functionality.

Steps to reproduce

On Site A:

1. Create custom block
2. Assign this custom block to any region
3. Export configuration of the site

On Site B:

1. Import configuration from site A
2. Go to Block layout and you will see custom block in correctly assigned region, however block won't work or actually show anything.
3. Edit that block, you will see this messages:
   • Block description Broken/Missing
This block is broken or missing. You may be missing content or you might need to enable the original module.
4. Go under Custom block library and you won't see custom block.

Block layout got exported but block content didn't resulting in broken relationship.

Suggested solution

Don't export custom blocks through Configuration management.