[FEATURE]: Disable everything related to comments

[jakewrfoster]

Description

It is extremely uncommon for Alley sites to utilize WordPress comments. They should be disabled by default.

Use Case

Most sites shouldn't include the cruft that Core comments add to the editorial experience. Disabling them by default makes things cleaner and simpler both from an editorial and a technological perspective.

[jakewrfoster]

Issue originally raised by @mboynes.

[vancoder]

I have WIP for this.

[kevinfodness]

From our hackathon today, a list of requirements:

• Comments should not be visible on the front end in a core theme (i.e., we should not be relying on suppressing the comments markup in a custom theme)
• Comments UI in the admin should be completely disabled, including on the sidebar, in the admin bar, in Gutenberg, and including Discussion settings (the entire Discussion menu area should be removed)
• Comments rewrite rules should be removed
• Comments REST API endpoints should be removed
• Comment data in REST API post responses should be removed
• Comments should be removed from feeds
• Pings and trackbacks should be disabled also (there is a separate feature for this, but that feature should be to ensure that pingbacks and trackbacks are disabled even if comments themselves are enabled, whereas this feature is to disable all types of comments entirely)
• Ensure feature test coverage

Further, we should remove post type support entirely for comments, including post types that have them built in from the start, such as post.