Review Entity Blocks module for core-readiness

Per #2850926: Add a block plugin that can display any viewable entity without a context → , we'd like to move Entity Blocks into core. For that to happen, there are a few things we need to do. And in case it does not happen, we'd end up with better code anyway.

To start, we need:

1. Solid test coverage.
2. No critical issues.
3. UI validation.

Read more here: What requirements must an experimental module meet? →