Encode non-ascii characters properly when sending 2fa emails

[kadamwhite]

Draft PR, not ready for merge yet. Attempted this as-is in #7 and had to revert due to an error:

Uncaught ValueError: mb_encode_mimeheader(): Argument #2 ($charset) must be a valid encoding, "3" given in /usr/src/app/vendor/humanmade/two-factor/providers/class.two-factor-email.php:151
Stack trace:
#0 /usr/src/app/vendor/humanmade/two-factor/providers/class.two-factor-email.php(151): mb_encode_mimeheader('Blo...', '3')
#1 /usr/src/app/vendor/humanmade/two-factor/providers/class.two-factor-email.php(171): Two_Factor_Email->generate_and_email_token(Object(WP_User))
#2 /usr/src/app/vendor/humanmade/two-factor/class.two-factor-core.php(361): Two_Factor_Email->authentication_page(Object(WP_User))
#3 /usr/src/app/vendor/humanmade/two-factor/class.two-factor-core.php(303): Two_Factor_Core::login_html(Object(WP_User), '0d498aca32a1c7d...', 'https://site...', '', Object(Two_Factor_Email))
#4 /usr/src/app/wordpress/wp-includes/class-wp-hook.php(307): Two_Factor_Core::backup_2fa('')
#5 /usr/src/app/wordpress/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters('', Array)
#6 /usr/src/app/wordpress/wp-includes/plugin.php(474): WP_Hook->do_action(Array)
#7 /usr/src/app/wordpress/wp-login.php(518): do_action('login_form_back...')
#8 {main}
  thrown

@ajvillegas mentioned,

The mb_encode_mimeheader function is expecting the charset as the second parameter, but I left the ENT_QUOTES flag from the wp_specialchars_decode we had previously on the PR and think this is what's causing the error. It looks like the second parameter should be UFT-8.

This PR makes that change.

[kadamwhite]

Limited testing indicates that this fixes the subject but not the sender field. Maybe we need to filter the get_option for blogname, to apply this within the option accessor?

add_filter( 'option_blogname', function( $blogname ) {
    return mb_encode_mimeheader( $blogname, 'UTF-8' );
}