Add Spotify Auth and Playlists

[overflew]

Best feature ever.

Spotify auth:

• User can complete OAuth w/ Spotify, so we can call Spotify API from this extension
• Slot this new functionality in somewhere thoughtful

Spotify Playlists:

• Be able browse the user's playlists
• Be able to queue songs from those playlists

[overflew]

Remaining work:

CSS: Pretty things up

• The 'logged in as' user header
• Playlist list items - fix up layout, hover state, etc
• Other... stuff?

I've created the sh-ffs style, as I'm not sure what default spacing Material is supposed to provide...

Things I'd like to fix:

• Ensure auth check: -- Currently, it's just Spotify Home page that checks the auth token, and re-auths if needed. -- Need to make it happen as part of routing -- E.g.: When reloading a page like playlist details, it won't re-auth properly...

Remaining features:

• Add 'Load more' button (for playlists w/over 100 songs)
• (?) Add a sort order for playlists (as some of mine work better by 'recently added'
• Replace Loading... text with sweet gfx from https://loading.io/ ?

[overflew]

NB: Will check the auth flows for if the user opens the Spotify sign in dialog, then cancels. Currently seeing: Cannot read property 'match' of undefined around:

window.chrome.identity.launchWebAuthFlow({
            'url': loginUrl,
            'interactive': !silentMode
        }, (requestUrl) => {
            let hashMatch = requestUrl.match('#.*');
            let hash = hashMatch[0].substring(1); // cut off leading #