Check for /booksonic when adding a server

[popeen]

The most used docker container has /booksonic at the end of the url. This have lead to confusion about setting up the server. When you enter just http;//ip:port you will get an error but if you try in the browser it works fine.

Add a check for this on the add server view that automatically checks this if login fail at http://ip:port and if it finds it at /booksonic automatically change it in a seamless way.