Comments: Add a capabilities object to comment types

[adamsilverstein]

Add capability_type / capabilities args to register_comment_type(), a
WP_Comment_Type::$cap object, and a get_comment_type_capabilities() helper
mirroring get_post_type_capabilities(). Advisory metadata only in this step:
map_meta_cap() is deliberately unchanged, so there is no behavior change.

Rebuilt from #52 as
part of a stacked-PR trial.

Trac ticket: https://core.trac.wordpress.org/ticket/35214

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