XHTML well-formedness: ampersand in building query

[brettz95]

In /wp-admin/edit-form-advanced.php, after making a submission, preview=true was set (line 48), but its unescaped ampersand caused an error in application/xhtml+xml . Following add_query_arg(), to /wp-includes/functions.php, and in turn to build_query() where the ampersand is added (and which goes to _http_build_query in /wp-includes/compat.php where the separator argument, here the ampersand, is used (unmodified) as the glue in an implode). XML requires ampersands to be escaped, so changing "&" to "&" (as in the patch) fixes the issue.

Patch fixes application/xhtml+xml issue

(In [14137]) Escape some permalinks. add_query_arg will &, not &, so we need to use esc_url those when used in an attribute. fixes #12126.