Prevent sending a 404 status when returning the response for the customize preview, since it causes the jQuery Ajax to fail. Send 200 instead.

props westonruter.
fixes #27992.
Built from https://develop.svn.wordpress.org/trunk@28970


git-svn-id: http://core.svn.wordpress.org/trunk@28759 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix texturization of quotes when enclosed by angle brackets.

props miqrogroove.
fixes #28718.
Built from https://develop.svn.wordpress.org/trunk@28971


git-svn-id: http://core.svn.wordpress.org/trunk@28760 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve oEmbed caching. Introduces the concept of a TTL for oEmbed caches and a filter for `oembed_ttl`.

We will no longer replace previously valid oEmbed responses with an `{{unknown}}` cache value. When this happens due to reaching a rate limit or a service going down, it is data loss, and is not acceptable. This means that oEmbed caches for a post are no longer deleted indiscriminately every time that post is saved.

oEmbed continues to be cached in post meta, with the addition of a separate meta key containing the timestamp of the last retrieval, which is used to avoid re-requesting a recently cached oEmbed response. By default, we consider a valued cached in the past day to be fresh. This can greatly reduce the number of outbound requests, especially in cases where a post containing multiple embeds is saved frequently.

The TTL used to determine whether or not to request a response can be filtered using `oembed_ttl`, thus allowing for the possibility of respecting the optional oEmbed response parameter `cache_age` or altering the period of time a cached value is considered to be fresh.

Now that oEmbeds are previewed in the visual editor as well as the media modal, oEmbed caches are often populated before a post is saved or published. By pre-populating and avoiding having to re-request that response, we also greatly reduce the chances of a stampede happening when a published post is visible before oEmbed caching is complete.

As it previously stood, a stampede was extremely likely to happen, as the AJAX caching was only triggered when `$_GET['message']` was 1. The published message is 6. We now trigger the caching every time `$_GET['message']` is present on the edit screen, as we are able to avoid triggering so many HTTP requests overall.

props markjaquith. fixes #14759. see #17210.

Built from https://develop.svn.wordpress.org/trunk@28972


git-svn-id: http://core.svn.wordpress.org/trunk@28761 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Correct typo in `sanitize_html_class()` docblock.
Built from https://develop.svn.wordpress.org/trunk@28973


git-svn-id: http://core.svn.wordpress.org/trunk@28762 1a063a9b-81f0-0310-95a4-ce76da25c4cd