create SpotifyController

and have SessionController and UsersController inherit
from it.
move functions from helper into controller
rails g controller playlists
start of playlists_controller#show
get_playlist_tracks
users#show lists user's playlists
redirect to users@show after logging in
playlists#show lists all the tracks
add :current_user to session
method get_user_tracks
if #show-ing current_user, then get their tracks
oops, need more permissions to get_user_tracks
refresh access_token if it has expired
better nil handling in playlists@#show
better nil handling on users#show
clear out default readme
we want permission to modify public playlists