Don't render comments on pages with comments disabled

[Sentynel]

The README suggests using the following in a custom theme:

if ( comments_open() ) {
	coral_talk_comments_template();
}

Without a custom theme, the code instead uses the WordPress comments_template filter. However, there's no check in this logic for comments_open(), so comments get rendered on pages with comments disabled.

Additionally, when a draft post is previewed, Talk comments are rendered. This leads to a few issues with autopopulating the assets record, primarily that WordPress doesn't typically generate the pretty URL as the canonical URL - that is, the "canonical" URL will be yourdomain.com/?p=1234 rather than yourdomain.com/post-name/. This leads to multiple asset records, one for the draft version of the post and one for the published version.

The solution is a simple check for both of these states in the filter code.

Testing: comments are still rendered for published posts with comments enabled; comments are not rendered for posts with comments disabled, or for previews of draft posts.