Fix so that approved examples are publicly viewable

* Make examples publicly viewable, if present
* Hook 'comments_open' to only enable submitting examples by site members
* Add hook 'wporg_devhub-can_user_post_example' to allow overriding default post commenting status
* Temporarily disable any non-example commenting
* Rename wporg_developer_comment() to more appropriate wporg_developer_example()
* Remove unnecessary code-example template
Permit examples to contain any markup

* Add post_type_supports_examples()
* Permit examples to have any content
* Fully escape content of examples via 'get_comment_text'
Fixes for, and selective activation of, handbooks

* Landing page:
  * Permit members of site to access the page
  * Set proper links to handbooks
  * Don't display the page content
* Redirect naked '/handbook/' requests to home page
* Remove 'Handbook' from handbook breadcrumbs
* Base site section title for handbooks on is_post_type_archive()
Add watch/unwatch post action link