Admin settings pages Best Practices update

* Applied a number of settings form best practices, such as:
 - use of `screen_icon()` function in combination with the appropriate css
 - use of `get_admin_page_title()` function in combination with ensuring that the correct title is registered when the submenu page is added.
 - use of the `options-head.php` file for automagically displaying the updated/error messages on settings save.
 - use of `submit_button()` function
* Improved form support for UTF-8 by adding the `accept-charset="utf-8"` property
* Corrected faulty multisite option registration - everywhere in the code the option is referred to as `wpseo_ms`, except for in `register_setting()`. This could cause issues.