Add option to globally disable comments on a content type.

Describe your issue or idea

Give administrators the ability to globally disable comments for a content type. Then content editors do not have the Comment settings vertical tab for that content type.

Steps to reproduce (if reporting a bug)

• visit /admin/structure/types/manage/TYPE

Actual behavior (if reporting a bug)

• There is no setting to entirely disable comments.
• There is just a setting to close comments.

Desired behavior (if reporting a bug)

• Add a setting to globally disable comments across the content type. Meaning the content editors will not have access to the comment settings to turn them on or off per node.

Relevant version/system information (if applicable)

1.x

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PR: https://github.com/backdrop/backdrop/pull/2117