Added link to logo image using $config['skin_logo'] (#8509)

Added ability to define a link URL for the logo image using $config['skin_logo'].
Works by defining the logo type for $config['skin_logo'] as [link] for example:

    $config['skin_logo'] = ['elastic:*[link]' => 'https://www.example.com'];
    $config['skin_logo'] = ['[link]' => 'https://www.example.com'];

Same relative path rules apply as when you define location of logo image file.
Starting with http:// or https:// is absolute.
Starting with a slash /page.html is relative to the skin directory ie example.com/skins/elastic/page.html
Starting without a slash page.html is relative to the website root directory ie example.com/page.html