Add the current theme compat id to the body class array

[hnla]

It might be helpful to add the theme_compat->theme->id to the body classes as an additional hook for authors, plugins to be able to hook styles to particular template packs.

We can do this now in advance of new registered themes safely as we'll just be adding a class that wouldn't exist before.

I've used the id in stead of the name as 'name' in legacies case is a string with whitespace, instead I've prepended bp- to the id to create the class token.

[hnla]

Add theme_compat id to body classes.

[hnla]

In 11605:

Add theme compat id to body classes

With the advent of selectable template packs it might help to identify theme in the body classes.

Commit adds the bp_get_theme_compat_id() appended to string 'bp-', using name can result in white space string.

Fixes #7537