Migrate from Soundcloud JS wrapper to manual OAuth management

[benbelow]

Chrome on desktop handles certain popups by opening a new tab instead of a popup. This means that on redirect finishing, the page is a new version of the tab and doesn't remember it should be requesting something.

Two ways around this:

(1) have the state of the page remember the user's OAuth token and keep them signed in across visits

(PROBABLY A GOOD IDEA ANYWAY!)

although it means the first time you try to make a playlist, you'll instead login and start again on a duplicate tab

(2) follow https://stackoverflow.com/questions/44815559/chrome-how-to-open-popup-in-new-window-instead-of-new-tab-20170629-chrome-59-0 to get the popup to open as a popup proper, instead of a new tab

[benbelow]

N.B. (1) will require in session memory, which means a backend. Might mean I need to re-think hosting strategy - look into firebase options for a backend express / mongo setup