VBO: Allow bulk enabling/disabling comments on existing content.

This is part of #3797

In #764 we moved the "Hidden" option for comments from the content type level to the node level. So if one wants to achieve the previous functionality (for example to set the comments for all content of a specific content type to hidden), then the only way to do it would be to filter content by this content type and then set their comments to hidden en masse. But we don't have such an option available. Let's provide this VBO then.

Here's a rundown of the problem:

• create a vanilla Backdrop site
• start adding content
• after some time, a decision is made to disable comments for the Post content type + hide all comments that may have been added for existing content
• you configure the Post content type so that comments won't be allowed for new content by default
• there is no easy way to close/hide comments for existing content of the type post via the admin UI

Item from BAD Camp training, probably in VBO