Bootstrap/Load: Make handling the `/favicon.ico` requests more flexible.

Previously, `wp_favicon_request()` was introduced in [13205] to avoid a performance hit of serving a full 404 page on every favicon request.

While working as intended, that implementation did not provide a way for theme or plugin authors to manage the behavior of favicon requests.

This changeset implements the following logic (only applied if WordPress is installed in the root directory):

* If there is a Site Icon set in Customizer, redirect `/favicon.ico` requests to that icon.
* Otherwise, use the WordPress logo as a default icon.
* If a physical `/favicon.ico` file exists, do nothing, let the server handle the request.

Handling `/favicon.ico` is now more consistent with handling `/robots.txt` requests.

New functions and hooks:

* Introduce `is_favicon()` conditional tag to complement `is_robots()`.
* Introduce `do_favicon` action to complement `do_robots` and use it in template loader.
* Introduce `do_favicon()` function, hooked to the above action by default, to complement `do_robots()`.
* Introduce `do_faviconico` action to complement `do_robotstxt`, for plugins to override the default behavior.
* Mark `wp_favicon_request()` as deprecated in favor of `do_favicon()`.

Props jonoaldersonwp, birgire, joostdevalk, mukesh27, SergeyBiryukov.
Fixes #47398.
Built from https://develop.svn.wordpress.org/trunk@47018


git-svn-id: https://core.svn.wordpress.org/trunk@46818 1a063a9b-81f0-0310-95a4-ce76da25c4cd