Deprecate the `blog_details` filter

[flixos90]

Per discussion during multisite office-hours (​https://wordpress.slack.com/archives/core-multisite/p1477413103001760) we are going to deprecate the blog_details filter and advise developers to use site_details instead.

One issue with that is since we have been replacing all the get_blog_details() calls in Core with get_site() (see #37102), the blog_details filter isn't run at all in some of these occasions. Therefore we will also add the blog_details filter (as deprecated) to WP_Site::get_details() to maintain backward compatibility.

The function get_blog_details() will also be deprecated very soon, likely in the next release.

Because of the complexity of these function replacements and the deprecated filter we need to write a dev-note on this or at least make it a highlighted topic on the general multisite dev-note.

[flixos90]

Status of this and the related tickets:

• #38351 and #38355 will be committed later today.
• I will work on a patch for this and #38350 now.

[flixos90]

38491.diff​ deprecates the blog_details filter and makes it available in WP_Site::get_details() to maintain BC.

It also replaces get_blog_details() with get_site() in all unit tests that do not test get_blog_details() specifically.

Note: #38350, #38351 and #38355 should probably go in before this one.

[jeremyfelt]

Replying to flixos90:

38491.diff​ deprecates the blog_details filter and makes it available in WP_Site::get_details() to maintain BC.

Looking good. apply_filters_deprecated() already runs has_filter() as its first line, so we can probably skip that part.

It also replaces get_blog_details() with get_site() in all unit tests that do not test get_blog_details() specifically.

Let's handle this separately as part of #37102.

[flixos90]

38491.2.diff​ removes the unnecessary has_filter() check and also gets rid of unit test replacements which will be handled separately.

[jeremyfelt]

In 38936:

Multisite: Deprecate the blog_details filter.

get_site() has replaced get_blog_details() throughout core and is the recommended way to retrieve a current or single site object.

The blog_details filter is applied when full details are requested from get_blog_details().

To ensure backwards compatibility in the switch to get_site(), this blog_details filter is now applied in WP_Site::get_details() and marked as deprecated with a note to rely instead on the site_details filter introduced in 4.6.

Props flixos90.
See #37102.
Fixes #38491.

[desrosj]

Linking the dev note that was published here for reference: ​https://make.wordpress.org/core/2016/11/04/multisite-focused-changes-in-4-7/