Add a rule on block machine name

Problem/Motivation

When placing block on a page, you can set the block machine name. But the configuration of this block depends on the theme you place your block in.

On the same websites, if there are multiple themes, you can't place the same block on another theme with the same machine name because it is already used.

Proposed resolution

A best practice would be to prefix block machine name by the machine name of the theme it is in.

For example: block.block.bartik_help.yml

Remaining tasks

• Review patch
• Update core umami and testing modules