Changing Create Song from a JS prompt to a sliding navbar form

* Mainly to remove standard JS system message formatting (but also to look cool)
* Adding Create Song image button
* Updated login form listener to be more specific to remove confusion with create song form
* Removed navbar gradient css as no longer needed
* Removed Musician css tag as no longer needed
* Greatly simplified checkName function since create song form is now the same as create user form (both in page), so no longer need to differentiate between them (another reason for switching)