Add wp-admin body classes for wp_version and admin_color

[andy]

Some plugins (e.g. WordPress.com Stats, Jetpack) display in their wp-admin pages sections of HTML generated remotely. As the styling of this HTML is affected by both WP version and the user's admin color scheme, these plugins need to modify their HTML to include CSS fixes dependent on those parameters. Rather than send the parameters for dynamic style generation, we should add body classes specifying the WP version and color scheme.

Attached patch adds body classes to wp-admin.

[nacin]

Should classes start with a letter? And are dots allowed, in the sense that they're thus tough to target in CSS? I can't remember.

So for 3.2.1, this would produce classes of 3, 3.2, and 3.2.1. The first one at the very least seems a bit excessive. Perhaps we can at least prefix/suffix them? branch-3-2 and version-3-2-1?

[andy]

Updated patch to remove first one and add prefixes.

[nacin]

$user_id isn't set here, so I'm going to let get_user_option() do the lifting there.

[nacin]

In [17957]:

Add admin body classes for the WP branch, WP version, and current admin color. props andy, fixes #17496.