Possible Spotify access token race conditions

[samcal]

Since most of our code is running async, we need to be super careful about what the current state of the spotify api singleton is.

[samcal]

In routes/submit_route.js, we call setTokens before getCurrentUser. Then, however, we do more stuff inside the Promise continuation block with the SpotifyService, assuming that it will use the same tokens as before. This is, however, not the case if there are two different parties making calls with the Spotify API singleton, as their respective access tokens will be mixed up.