Important security fixes, bugfixes, style fixes

Users can no longer register a new user if they are logged in.
User can no longer hack the modal to log them in if they are logged in.
Moved playlistPicker from header to song page, to fix occuring errors.
Fixed generating invalid content for your playlists in the sidebar
The playlist modal can no longer be hacked to add playlists without
permission.
Upload modal can no longer be hacked to upload songs without user
permission.
Form tab for modifying user data can no longer be hacked to manipulate
user data from other users.
Fixed styling from jplayer. Jplayer progress bar was on top of header
instead of underneath the header while scrolling the page.