* Deprecate `login_headertitle` filter, introduce `login_headertext` as a replacement.
* For backwards compatibility, if a `login_headertitle` is set, it will be used as link text.
* Make the login header logo URL and text consistent between single site and Multisite.
* Avoid ambiguity of where the WordPress logo points to; link to WordPress.org by default.
* `login_headerurl` filter is still available to change the URL of the header logo.
Props afercia, pratikkry, chetan200891.
Fixes#42537.
git-svn-id: https://develop.svn.wordpress.org/trunk@44899 602fd350-edb4-49c9-b593-d223f7449a82