Song limits feature

-  Limits total number of songs in playlist to 20.  Could be 30, 40, 50, whatever.  It's an easy thing to adjust (line 16 of playlist.rb)

-  Limits number of songs a user can submit.  This is based off the total number of songs in the list submitted by that user.  Right now it's set so a user can't add more than four songs out of the total 20.  Once again it's an easy change to make if we decide a user should be able to add more or less (line 25 of playlist.rb)